diff --git a/README.md b/README.md
index f391024f4..45db3aa51 100644
--- a/README.md
+++ b/README.md
@@ -34,7 +34,7 @@ Zen is a firefox-based browser with the aim of pushing your productivity to a ne
 
 ### Firefox Versions
 
-- [`Release`](https://zen-browser.app/download) - Is currently built using Firefox version `154.0.1`!
+- [`Release`](https://zen-browser.app/download) - Is currently built using Firefox version `155.0`!
 - [`Twilight`](https://zen-browser.app/download?twilight) - Is currently built using Firefox version `RC 155.0`!
 
 ### Contributing
diff --git a/build/firefox-cache/l10n-last-commit-hash b/build/firefox-cache/l10n-last-commit-hash
index e668d9111..24e25043a 100644
--- a/build/firefox-cache/l10n-last-commit-hash
+++ b/build/firefox-cache/l10n-last-commit-hash
@@ -1 +1 @@
-6b4abe0093721c1be89eb41b3fb060dd6336923d
\ No newline at end of file
+20e5abbb642ab21d943bc1261864d9c531ba421a
\ No newline at end of file
diff --git a/src/external-patches/firefox/bug_2011236.patch b/src/external-patches/firefox/bug_2011236.patch
index 1a46a9c03..f853cfc36 100644
--- a/src/external-patches/firefox/bug_2011236.patch
+++ b/src/external-patches/firefox/bug_2011236.patch
@@ -150,8 +150,8 @@ diff --git a/docshell/shistory/SessionHistoryEntry.cpp b/docshell/shistory/Sessi
            aLoadState->TypeHint())) {
    MOZ_DIAGNOSTIC_ASSERT(!mURI->SchemeIs("javascript"));
  
-   // Pull the upload stream off of the channel instead of the load state, as
-@@ -124,11 +133,12 @@
+   // Verify the documented purpose of mBaseURI.
+@@ -130,11 +139,12 @@
    loadInfo->GetPrincipalToInherit(
        getter_AddRefs(mSharedState.Get()->mPrincipalToInherit));
  
diff --git a/src/external-patches/firefox/issue_14710.patch b/src/external-patches/firefox/issue_14710.patch
index 54e288ed8..da0f7160e 100644
--- a/src/external-patches/firefox/issue_14710.patch
+++ b/src/external-patches/firefox/issue_14710.patch
@@ -1,7 +1,24 @@
 diff --git a/toolkit/components/pictureinpicture/PictureInPicture.sys.mjs b/toolkit/components/pictureinpicture/PictureInPicture.sys.mjs
 --- a/toolkit/components/pictureinpicture/PictureInPicture.sys.mjs
 +++ b/toolkit/components/pictureinpicture/PictureInPicture.sys.mjs
-@@ -119,10 +119,19 @@
+@@ -39,10 +39,16 @@
+ const TOGGLE_POSITION_PREF =
+   "media.videocontrols.picture-in-picture.video-toggle.position";
+ const TOGGLE_POSITION_RIGHT = "right";
+ const TOGGLE_POSITION_LEFT = "left";
+ const RESIZE_MARGIN_PX = 16;
++// Close reasons that should suppress auto-toggle when the originating tab is backgrounded
++const DELIBERATE_CLOSE_REASONS = [
++  "CloseButton",
++  "ClosePlayerShortcut",
++  "Shortcut",
++];
+ 
+ XPCOMUtils.defineLazyPreferenceGetter(
+   lazy,
+   "PIP_ENABLED",
+   "media.videocontrols.picture-in-picture.enabled",
+@@ -119,10 +125,19 @@
        }
        case "PictureInPicture:VideoTabHidden": {
          if (!lazy.PIP_ENABLED || !lazy.PIP_WHEN_SWITCHING_TABS) {
@@ -21,12 +38,13 @@ diff --git a/toolkit/components/pictureinpicture/PictureInPicture.sys.mjs b/tool
            break;
          }
          let actor = browsingContext.currentWindowGlobal.getActor(
-@@ -261,10 +270,12 @@
+@@ -261,10 +276,13 @@
    maxConcurrentPlayerCount: 0,
  
    // Maps auto pip browser to PictureInPictureParent actor
    weakAutoPipBrowserToParent: new WeakMap(),
  
++  // Browsers with deliberately closed PiP windows; suppresses auto-toggle.
 +  weakAutoPipBrowserClosedDeliberately: new WeakSet(),
 +
    /**
@@ -34,7 +52,28 @@ diff --git a/toolkit/components/pictureinpicture/PictureInPicture.sys.mjs b/tool
     *
     * @param {PictureInPictureParent} pipActorRef
     *   Reference to the calling PictureInPictureParent actor
-@@ -899,10 +910,17 @@
+@@ -441,10 +459,20 @@
+   },
+ 
+   onPipSwappedBrowsers(event) {
+     let otherTab = event.detail;
+     if (otherTab) {
++      if (
++        this.weakAutoPipBrowserClosedDeliberately.has(
++          event.target.linkedBrowser
++        )
++      ) {
++        this.weakAutoPipBrowserClosedDeliberately.add(otherTab.linkedBrowser);
++        this.weakAutoPipBrowserClosedDeliberately.delete(
++          event.target.linkedBrowser
++        );
++      }
+       for (let win of Services.wm.getEnumerator(WINDOW_TYPE)) {
+         if (this.weakWinToBrowser.get(win) === event.target.linkedBrowser) {
+           this.weakWinToBrowser.set(win, otherTab.linkedBrowser);
+           this.removePiPBrowserFromWeakMap(event.target.linkedBrowser);
+           this.removeOriginatingWinFromWeakMap(event.target.linkedBrowser);
+@@ -899,10 +927,20 @@
      const { reason, actorRef } = closeData;
      const win = this.getWeakPipPlayer(actorRef);
      if (!win) {
@@ -43,8 +82,11 @@ diff --git a/toolkit/components/pictureinpicture/PictureInPicture.sys.mjs b/tool
 +
 +    const browser = this.weakWinToBrowser.get(win);
 +
-+    if (reason === "CloseButton") {
-+      PictureInPicture.weakAutoPipBrowserClosedDeliberately.add(browser);
++    if (browser && DELIBERATE_CLOSE_REASONS.includes(reason)) {
++      const tabbrowser = browser.getTabBrowser();
++      if (tabbrowser && tabbrowser.selectedBrowser != browser) {
++        PictureInPicture.weakAutoPipBrowserClosedDeliberately.add(browser);
++      }
 +    }
 +
      this.removePiPBrowserFromWeakMap(this.weakWinToBrowser.get(win));
@@ -52,4 +94,69 @@ diff --git a/toolkit/components/pictureinpicture/PictureInPicture.sys.mjs b/tool
  
      Glean.pictureinpicture["closedMethod" + reason].record();
      await this.closePipWindow(win);
+diff --git a/toolkit/components/pictureinpicture/tests/browser_autotoggle.js b/toolkit/components/pictureinpicture/tests/browser_autotoggle.js
+--- a/toolkit/components/pictureinpicture/tests/browser_autotoggle.js
++++ b/toolkit/components/pictureinpicture/tests/browser_autotoggle.js
+@@ -297,5 +297,61 @@
+       await new Promise(resolve => setTimeout(resolve, 2000));
+       assertNoPiPWindowsOpen();
+     }
+   );
+ });
++
++/**
++ * Tests that closing a PiP window deliberately (via Shift+ESC) while the tab is backgrounded
++ * suppresses auto-toggle, but auto-toggle works normally on the next background.
++ */
++add_task(async function autopip_suppress_on_deliberate_close() {
++  await SpecialPowers.pushPrefEnv({
++    set: [[ALWAYS_SHOW_PREF, true]],
++  });
++
++  // Open a new window and save a handle for the first tab
++  let win1 = await BrowserTestUtils.openNewBrowserWindow();
++  let firstTab = win1.gBrowser.selectedTab;
++
++  // Open a new tab containing a video
++  let pipTab = await BrowserTestUtils.openNewForegroundTab(
++    win1.gBrowser,
++    TEST_PAGE
++  );
++  let browser = pipTab.linkedBrowser;
++
++  // Ensure the video is playing
++  let videoID = "with-controls";
++  await ensureVideosReady(browser);
++  await SpecialPowers.spawn(browser, [videoID], async videoID => {
++    await content.document.getElementById(videoID).play();
++  });
++
++  // Switch from the video tab to the first tab and check if the PiP window opened
++  let domWindowOpened = BrowserTestUtils.domWindowOpenedAndLoaded(null);
++  await BrowserTestUtils.switchTab(win1.gBrowser, firstTab);
++  let pipWin = await domWindowOpened;
++  ok(pipWin, "PiP window automatically opened.");
++
++  // Close the PiP window deliberately while the tab is backgrounded and check that it doesn't reopen
++  domWindowOpened = BrowserTestUtils.domWindowOpenedAndLoaded(null);
++  // A short delay is needed to make the test more robust
++  // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
++  await new Promise(resolve => setTimeout(resolve, 2000));
++  EventUtils.synthesizeKey("KEY_Escape", { shiftKey: true }, pipWin);
++  let reopened = await Promise.race([
++    domWindowOpened.then(() => true),
++    // eslint-disable-next-line mozilla/no-arbitrary-setTimeout
++    new Promise(resolve => setTimeout(() => resolve(false), 2000)),
++  ]);
++  ok(!reopened, "PiP window should not reopen after deliberate close");
++
++  // Switch to the video tab and back to the first tab, which should automatically open the PiP window
++  domWindowOpened = BrowserTestUtils.domWindowOpenedAndLoaded(null);
++  await BrowserTestUtils.switchTab(win1.gBrowser, pipTab);
++  await BrowserTestUtils.switchTab(win1.gBrowser, firstTab);
++  pipWin = await domWindowOpened;
++  ok(pipWin, "PiP window automatically opened.");
++
++  await BrowserTestUtils.closeWindow(win1);
++});
 
diff --git a/src/zen/tests/mochitests/backforward/browser_history_menu.js b/src/zen/tests/mochitests/backforward/browser_history_menu.js
index 8623d5405..a0cff08b1 100644
--- a/src/zen/tests/mochitests/backforward/browser_history_menu.js
+++ b/src/zen/tests/mochitests/backforward/browser_history_menu.js
@@ -44,7 +44,7 @@ async function openHistoryMenu(useContextMenu) {
 async function testBackForwardMenu(useContextMenu) {
   let tab = await BrowserTestUtils.openNewForegroundTab(
     gBrowser,
-    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    // eslint-disable-next-line sdl/no-insecure-url
     "http://example.com"
   );
 
@@ -90,7 +90,7 @@ async function testBackForwardMenu(useContextMenu) {
     );
 
     let node = popupEvent.target.lastElementChild;
-    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    // eslint-disable-next-line sdl/no-insecure-url
     is(node.getAttribute("uri"), "http://example.com/", "'1' item uri");
     is(node.getAttribute("index"), "0", "'1' item index");
     is(
@@ -100,7 +100,7 @@ async function testBackForwardMenu(useContextMenu) {
     );
 
     node = node.previousElementSibling;
-    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    // eslint-disable-next-line sdl/no-insecure-url
     is(node.getAttribute("uri"), "http://example.com/2.html", "'2' item uri");
     is(node.getAttribute("index"), "1", "'2' item index");
     is(
@@ -111,7 +111,7 @@ async function testBackForwardMenu(useContextMenu) {
 
     if (iter >= 3) {
       node = node.previousElementSibling;
-      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      // eslint-disable-next-line sdl/no-insecure-url
       is(node.getAttribute("uri"), "http://example.com/3.html", "'3' item uri");
       is(node.getAttribute("index"), "2", "'3' item index");
       is(
@@ -133,7 +133,7 @@ async function testBackForwardMenu(useContextMenu) {
     } else {
       let newTabPromise = BrowserTestUtils.waitForNewTab(
         gBrowser,
-        // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+        // eslint-disable-next-line sdl/no-insecure-url
         url => url == "http://example.com/"
       );
 
diff --git a/src/zen/tests/mochitests/caps/browser_principalSerialization_json.js b/src/zen/tests/mochitests/caps/browser_principalSerialization_json.js
index f79f269fd..bdc34988b 100644
--- a/src/zen/tests/mochitests/caps/browser_principalSerialization_json.js
+++ b/src/zen/tests/mochitests/caps/browser_principalSerialization_json.js
@@ -88,32 +88,32 @@ add_task(async function test_contentPrincipal() {
   */
   let tests = [
     {
-      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      // eslint-disable-next-line sdl/no-insecure-url
       input: { uri: "http://example.com/", OA: {} },
       expected: `{"${contentId}":{"${content}":"http://example.com/"}}`,
     },
     {
-      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      // eslint-disable-next-line sdl/no-insecure-url
       input: { uri: "http://mozilla1.com/", OA: {} },
       expected: `{"${contentId}":{"${content}":"http://mozilla1.com/"}}`,
     },
     {
-      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      // eslint-disable-next-line sdl/no-insecure-url
       input: { uri: "http://mozilla2.com/", OA: { userContextId: 0 } },
       expected: `{"${contentId}":{"${content}":"http://mozilla2.com/"}}`,
     },
     {
-      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      // eslint-disable-next-line sdl/no-insecure-url
       input: { uri: "http://mozilla3.com/", OA: { userContextId: 2 } },
       expected: `{"${contentId}":{"${content}":"http://mozilla3.com/","${suffix}":"^userContextId=2"}}`,
     },
     {
-      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      // eslint-disable-next-line sdl/no-insecure-url
       input: { uri: "http://mozilla4.com/", OA: { privateBrowsingId: 1 } },
       expected: `{"${contentId}":{"${content}":"http://mozilla4.com/","${suffix}":"^privateBrowsingId=1"}}`,
     },
     {
-      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      // eslint-disable-next-line sdl/no-insecure-url
       input: { uri: "http://mozilla5.com/", OA: { privateBrowsingId: 0 } },
       expected: `{"${contentId}":{"${content}":"http://mozilla5.com/"}}`,
     },
diff --git a/src/zen/tests/mochitests/readermode/browser_bug1780350_readerModeSaveScroll.js b/src/zen/tests/mochitests/readermode/browser_bug1780350_readerModeSaveScroll.js
index 76add5511..dedeee53d 100644
--- a/src/zen/tests/mochitests/readermode/browser_bug1780350_readerModeSaveScroll.js
+++ b/src/zen/tests/mochitests/readermode/browser_bug1780350_readerModeSaveScroll.js
@@ -1,4 +1,4 @@
-/* eslint-disable @microsoft/sdl/no-insecure-url */
+/* eslint-disable sdl/no-insecure-url */
 /* Any copyright is dedicated to the Public Domain.
    http://creativecommons.org/publicdomain/zero/1.0/ */
 "use strict";
diff --git a/src/zen/tests/mochitests/sessionstore/browser.toml b/src/zen/tests/mochitests/sessionstore/browser.toml
index f6ab3d6e8..bcc9ff324 100644
--- a/src/zen/tests/mochitests/sessionstore/browser.toml
+++ b/src/zen/tests/mochitests/sessionstore/browser.toml
@@ -16,8 +16,6 @@ support-files = [
   "browser_frame_history_c2.html",
   "browser_formdata_format_sample.html",
   "browser_sessionHistory_slow.sjs",
-  "browser_policy_container_sample.html",
-  "browser_policy_container_sample_frame.html",
   "browser_scrollPositions_sample.html",
   "browser_scrollPositions_sample2.html",
   "browser_scrollPositions_sample_frameset.html",
@@ -249,8 +247,6 @@ skip-if = [
   "ccov", # Bug 1625525
 ]
 
-["browser_policy_container_not_stored.js"]
-
 ["browser_privatetabs.js"]
 
 ["browser_purge_domaindata.js"]
@@ -355,6 +351,7 @@ skip-if = [
 
 ["browser_sizemodeBeforeMinimized.js"]
 skip-if = [
+  "os == 'linux' && os_version == '22.04' && arch == 'x86_64' && opt && nova", # Bug 2057856
   "os == 'win' && os_version == '11.26200' && asan", # Bug 2019745
 ]
 
@@ -436,6 +433,7 @@ disabled="Disabled by import_external_tests.py"
 skip-if = [
   "os == 'mac' && os_version == '15.30' && arch == 'aarch64'", # Bug 2023967
 ]
+
 ["browser_tab_label_during_restore.js"]
 disabled="Disabled by import_external_tests.py"
 https_first_disabled = true
diff --git a/src/zen/tests/mochitests/sessionstore/browser_bfcache_telemetry.js b/src/zen/tests/mochitests/sessionstore/browser_bfcache_telemetry.js
index c1e987750..9e5d2bd36 100644
--- a/src/zen/tests/mochitests/sessionstore/browser_bfcache_telemetry.js
+++ b/src/zen/tests/mochitests/sessionstore/browser_bfcache_telemetry.js
@@ -1,44 +1,26 @@
 const URL1 = "data:text/html;charset=utf-8,<body><p>Hello1</p></body>";
 const URL2 = "data:text/html;charset=utf-8,<body><p>Hello2</p></body>";
 
-async function getBFCacheComboTelemetry(probeInParent) {
-  let bfcacheCombo;
-  await TestUtils.waitForCondition(() => {
-    let histograms;
-    if (probeInParent) {
-      histograms = Services.telemetry.getSnapshotForHistograms(
-        "main",
-        false /* clear */
-      ).parent;
-    } else {
-      histograms = Services.telemetry.getSnapshotForHistograms(
-        "main",
-        false /* clear */
-      ).content;
-    }
-    bfcacheCombo = histograms.BFCACHE_COMBO;
-    return bfcacheCombo;
-  });
-  return bfcacheCombo;
+async function getBFCacheComboValue(label) {
+  await Services.fog.testFlushAllChildren();
+  return Glean.bfcache.combo[label].testGetValue();
 }
 
-async function test_bfcache_telemetry(probeInParent) {
-  Services.telemetry.getHistogramById("BFCACHE_COMBO").clear();
+add_task(async () => {
+  await Services.fog.testFlushAllChildren();
+  Services.fog.testResetFOG();
 
   let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, URL1);
 
   BrowserTestUtils.startLoadingURIString(tab.linkedBrowser, URL2);
   await BrowserTestUtils.browserLoaded(tab.linkedBrowser);
 
-  let bfcacheCombo = await getBFCacheComboTelemetry(probeInParent);
+  await TestUtils.waitForCondition(
+    async () => (await getBFCacheComboValue("BFCache_Success")) !== null,
+    "Waiting for bfcache.combo 'BFCache_Success' to be recorded"
+  );
 
-  is(bfcacheCombo.values[0], 1, "1 bfcache success");
+  is(await getBFCacheComboValue("BFCache_Success"), 1, "1 bfcache success");
 
   gBrowser.removeTab(tab);
-}
-
-add_task(async () => {
-  await test_bfcache_telemetry(
-    Services.prefs.getBoolPref("fission.bfcacheInParent")
-  );
 });
diff --git a/src/zen/tests/mochitests/sessionstore/browser_oldformat.toml b/src/zen/tests/mochitests/sessionstore/browser_oldformat.toml
index 54842390e..348a0c004 100644
--- a/src/zen/tests/mochitests/sessionstore/browser_oldformat.toml
+++ b/src/zen/tests/mochitests/sessionstore/browser_oldformat.toml
@@ -153,6 +153,7 @@ skip-if = [
 
 ["browser_477657.js"]
 skip-if = [
+  "os == 'linux' && os_version == '22.04' && arch == 'x86_64' && opt && nova", # Bug 2057857
   "os == 'linux' && os_version == '24.04' && arch == 'x86_64' && display == 'x11'", # Bug 1610668
 ]
 
@@ -182,6 +183,9 @@ skip-if = [
 ]
 
 ["browser_524745.js"]
+skip-if = [
+  "os == 'linux' && os_version == '22.04' && arch == 'x86_64' && debug && nova", # Bug 2057858
+]
 
 ["browser_528776.js"]
 
diff --git a/src/zen/tests/mochitests/sessionstore/browser_policy_container_not_stored.js b/src/zen/tests/mochitests/sessionstore/browser_policy_container_not_stored.js
deleted file mode 100644
index 9258e3d54..000000000
--- a/src/zen/tests/mochitests/sessionstore/browser_policy_container_not_stored.js
+++ /dev/null
@@ -1,51 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
- * License, v. 2.0. If a copy of the MPL was not distributed with this
- * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-"use strict";
-
-/**
- * A session history entry only needs to retain the policy container for loads
- * that inherit their policies (about:blank, about:srcdoc, blob:, data:, ...).
- * Anything fetched over the network gets its policies from the response again,
- * so storing them would only bloat the session store (bug 2011236).
- */
-
-const TEST_ROOT = getRootDirectory(gTestPath).replace(
-  "chrome://mochitests/content",
-  "https://example.com"
-);
-const PARENT_URL = TEST_ROOT + "browser_policy_container_sample.html";
-const FRAME_URL = TEST_ROOT + "browser_policy_container_sample_frame.html";
-
-add_task(async function test_policy_container_only_for_inheriting_loads() {
-  const tab = BrowserTestUtils.addTab(gBrowser, PARENT_URL);
-  gBrowser.selectedTab = tab;
-  await promiseBrowserLoaded(tab.linkedBrowser, true, PARENT_URL);
-  await TabStateFlusher.flush(tab.linkedBrowser);
-
-  const state = JSON.parse(ss.getTabState(tab));
-  const topEntry = state.entries.at(-1);
-  is(topEntry.url, PARENT_URL, "collected the parent entry");
-
-  const children = topEntry.children ?? [];
-  const networkFrame = children.find(child => child.url === FRAME_URL);
-  const blankFrame = children.find(child => child.url === "about:blank");
-
-  ok(networkFrame, "collected the network-scheme subframe entry");
-  ok(blankFrame, "collected the about:blank subframe entry");
-
-  // The parent has a CSP, so both subframes inherit a policy container onto
-  // their load state. Only the one that cannot recover it from a response
-  // should keep it in session history.
-  ok(
-    !("policyContainer" in networkFrame),
-    "https subframe entry does not store a policyContainer"
-  );
-  ok(
-    "policyContainer" in blankFrame,
-    "about:blank subframe entry still stores its inherited policyContainer"
-  );
-
-  await BrowserTestUtils.removeTab(tab);
-});
diff --git a/src/zen/tests/mochitests/sessionstore/browser_policy_container_sample.html b/src/zen/tests/mochitests/sessionstore/browser_policy_container_sample.html
deleted file mode 100644
index 1b8ad8d29..000000000
--- a/src/zen/tests/mochitests/sessionstore/browser_policy_container_sample.html
+++ /dev/null
@@ -1,12 +0,0 @@
-<!DOCTYPE html>
-<html>
-<head>
-  <meta charset="utf-8">
-  <meta http-equiv="Content-Security-Policy" content="default-src * data: blob: 'unsafe-inline'">
-  <title>policyContainer session store sample</title>
-</head>
-<body>
-  <iframe id="network" src="browser_policy_container_sample_frame.html"></iframe>
-  <iframe id="blank" src="about:blank"></iframe>
-</body>
-</html>
diff --git a/src/zen/tests/mochitests/sessionstore/browser_policy_container_sample_frame.html b/src/zen/tests/mochitests/sessionstore/browser_policy_container_sample_frame.html
deleted file mode 100644
index 4675a0cd4..000000000
--- a/src/zen/tests/mochitests/sessionstore/browser_policy_container_sample_frame.html
+++ /dev/null
@@ -1,10 +0,0 @@
-<!DOCTYPE html>
-<html>
-<head>
-  <meta charset="utf-8">
-  <title>policyContainer session store sample frame</title>
-</head>
-<body>
-  frame
-</body>
-</html>
diff --git a/src/zen/tests/mochitests/sessionstore/browser_restored_window_features.js b/src/zen/tests/mochitests/sessionstore/browser_restored_window_features.js
index 20a1d7196..64aca7910 100644
--- a/src/zen/tests/mochitests/sessionstore/browser_restored_window_features.js
+++ b/src/zen/tests/mochitests/sessionstore/browser_restored_window_features.js
@@ -8,7 +8,6 @@ const BARPROP_NAMES = [
   "menubar",
   "personalbar",
   "scrollbars",
-  "statusbar",
   "toolbar",
 ];
 
@@ -45,7 +44,6 @@ add_task(async function testRestoredWindowFeatures() {
     menubar: true,
     personalbar: true,
     scrollbars: true,
-    statusbar: true,
     toolbar: true,
   };
   const TESTS = [
diff --git a/src/zen/tests/mochitests/sessionstore/browser_should_restore_tab.js b/src/zen/tests/mochitests/sessionstore/browser_should_restore_tab.js
index 07c699c31..318c846a6 100644
--- a/src/zen/tests/mochitests/sessionstore/browser_should_restore_tab.js
+++ b/src/zen/tests/mochitests/sessionstore/browser_should_restore_tab.js
@@ -18,8 +18,14 @@ async function check_tab_close_notification(openedTab, expectNotification) {
   }
   Services.obs.addObserver(topicObserver, NOTIFY_CLOSED_OBJECTS_CHANGED);
 
+  // (Bug 2058570) Intermittently, the tab state wasn't fully up to date before closing the tab.
+  // This caused test_navigated_about_home to think there was no meaningful history beyond about:home.
+  // Try flushing the tab state and then waiting for a notification if one is expected.
+  let flushedPromise = TabStateFlusher.flush(openedTab.linkedBrowser);
+
   BrowserTestUtils.removeTab(openedTab);
   await tabClosed;
+  await flushedPromise;
   // SessionStore does a setTimeout(notify, 0) to notifyObservers when it handles TabClose
   // We need to wait long enough to be confident the observer would have been notified
   // if it was going to be.
diff --git a/src/zen/tests/mochitests/sessionstore/marionette/test_restore_aiwindow.py b/src/zen/tests/mochitests/sessionstore/marionette/test_restore_aiwindow.py
index 530bfdecb..b899a392d 100644
--- a/src/zen/tests/mochitests/sessionstore/marionette/test_restore_aiwindow.py
+++ b/src/zen/tests/mochitests/sessionstore/marionette/test_restore_aiwindow.py
@@ -385,3 +385,64 @@ class TestSmartWindowDefaultManualRestore(
             tab_count,
             msg="Tab count should be preserved after restore",
         )
+
+
+class TestSmartWindowDefaultManualRestoreOverwritesNewTab(
+    SmartWindowDefaultMixin, SessionStoreTestCase
+):
+    """With Smart Window as default (startup.page=1), the startup window opens
+    showing the Smart new tab. Restoring a previously Smart session via
+    "Restore previous session" reuses that startup window (the types match),
+    and the restored tabs must *overwrite* the Smart new tab rather than being
+    appended alongside it."""
+
+    def test_smart_session_restored_overwrites_smart_new_tab(self):
+        self.marionette.execute_script(
+            """
+            Services.prefs.setBoolPref("browser.sessionstore.persist_closed_tabs_between_sessions", true);
+            """
+        )
+
+        self.wait_for_windows(
+            self.all_windows, "Not all requested windows have been opened"
+        )
+
+        # Make the saved session a Smart Window with the 3 content tabs from
+        # setUp (Tab 1/2/3), which are real pages, not the Smart new tab.
+        self.toggle_ai_window(True)
+        self.assertTrue(
+            self.is_ai_window(), msg="Window should be Smart before the restart"
+        )
+        tab_count = self.get_tab_count()
+        self.assertEqual(tab_count, 3, msg="Saved session should have 3 tabs")
+
+        self.marionette.quit()
+        self.marionette.start_session()
+        self.marionette.set_context("chrome")
+
+        # The new startup window opens Smart (Smart Window is the default) and
+        # shows a single Smart new tab.
+        self.assertTrue(
+            self.is_ai_window(),
+            msg="Startup window should open Smart when Smart Window is the default",
+        )
+
+        self.restore_last_session()
+
+        self.assertEqual(
+            len(self.marionette.chrome_window_handles),
+            1,
+            msg="Smart session should restore into the existing startup window",
+        )
+        self.assertTrue(
+            self.is_ai_window(),
+            msg="Restored window should stay Smart",
+        )
+
+        # Asserts if Smart new tab was overwritten, not appended to.
+        self.assertEqual(
+            self.get_tab_count(),
+            tab_count,
+            msg="Restored tabs should overwrite the Smart new tab, not be "
+            "appended alongside it",
+        )
diff --git a/src/zen/tests/mochitests/sessionstore/unit/test_write_json_length_hint.js b/src/zen/tests/mochitests/sessionstore/unit/test_write_json_length_hint.js
deleted file mode 100644
index fd4a44848..000000000
--- a/src/zen/tests/mochitests/sessionstore/unit/test_write_json_length_hint.js
+++ /dev/null
@@ -1,73 +0,0 @@
-/* Any copyright is dedicated to the Public Domain.
-   http://creativecommons.org/publicdomain/zero/1.0/ */
-
-"use strict";
-
-const { SessionWriter } = ChromeUtils.importESModule(
-  "resource:///modules/sessionstore/SessionWriter.sys.mjs"
-);
-
-const profd = do_get_profile();
-const { SessionFile } = ChromeUtils.importESModule(
-  "resource:///modules/sessionstore/SessionFile.sys.mjs"
-);
-
-const { updateAppInfo } = ChromeUtils.importESModule(
-  "resource://testing-common/AppInfo.sys.mjs"
-);
-updateAppInfo({
-  name: "SessionRestoreTest",
-  ID: "{230de50e-4cd1-11dc-8314-0800200c9a66}",
-  version: "1",
-  platformVersion: "",
-});
-
-add_setup(async function () {
-  let source = do_get_file("data/sessionstore_valid.js");
-  source.copyTo(profd, "sessionstore.js");
-  await writeCompressedFile(
-    SessionFile.Paths.clean.replace("jsonlz4", "js"),
-    SessionFile.Paths.clean
-  );
-  await SessionFile.read();
-});
-
-add_task(async function test_length_hint_updates_after_write() {
-  Assert.equal(
-    SessionWriter._jsonLengthHint,
-    0,
-    "Length hint starts at 0"
-  );
-
-  await SessionFile.write({});
-
-  let hintAfterSmall = SessionWriter._jsonLengthHint;
-  Assert.equal(
-    hintAfterSmall,
-    JSON.stringify({}).length,
-    "Hint matches the uncompressed JSON byte length"
-  );
-
-  let largerState = await IOUtils.readJSON(
-    PathUtils.join(do_get_cwd().path, "data", "sessionstore_complete.json")
-  );
-  await SessionFile.write(largerState);
-
-  Assert.greater(
-    SessionWriter._jsonLengthHint,
-    hintAfterSmall,
-    "Hint grows after writing a larger state"
-  );
-});
-
-add_task(async function test_length_hint_resets_on_wipe() {
-  await SessionFile.write({ windows: [{ tabs: [{ entries: [] }] }] });
-  Assert.greater(SessionWriter._jsonLengthHint, 0, "Hint is nonzero");
-
-  await SessionFile.wipe();
-  Assert.equal(
-    SessionWriter._jsonLengthHint,
-    0,
-    "Hint resets to 0 after wipe"
-  );
-});
diff --git a/src/zen/tests/mochitests/sessionstore/unit/xpcshell.toml b/src/zen/tests/mochitests/sessionstore/unit/xpcshell.toml
index 01b0fe1f2..661a6046d 100644
--- a/src/zen/tests/mochitests/sessionstore/unit/xpcshell.toml
+++ b/src/zen/tests/mochitests/sessionstore/unit/xpcshell.toml
@@ -41,8 +41,3 @@ skip-if = [
 ]
 
 ["test_startup_session_async.js"]
-
-["test_write_json_length_hint.js"]
-support-files = [
-  "data/sessionstore_complete.json",
-]
diff --git a/src/zen/tests/mochitests/shell/unit/test_isOneClickSetDefaultEnabled.js b/src/zen/tests/mochitests/shell/unit/test_isOneClickSetDefaultEnabled.js
new file mode 100644
index 000000000..639d62ce8
--- /dev/null
+++ b/src/zen/tests/mochitests/shell/unit/test_isOneClickSetDefaultEnabled.js
@@ -0,0 +1,174 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+ChromeUtils.defineESModuleGetters(this, {
+  ShellService: "moz-src:///browser/components/shell/ShellService.sys.mjs",
+  sinon: "resource://testing-common/Sinon.sys.mjs",
+});
+
+const USER_CHOICE_PREF = "browser.shell.setDefaultBrowserUserChoice";
+const REG_RENAME_PREF = "browser.shell.setDefaultBrowserUserChoice.regRename";
+
+function mockNativeShellService(overrides) {
+  const mock = {
+    QueryInterface: ChromeUtils.generateQI([Ci.nsIWindowsShellService]),
+    checkAllProgIDsExist: () => true,
+    checkBrowserUserChoiceHashes: () => true,
+    isDefaultBrowser: () => false,
+    isUserChoiceProtectionDriverRunning: () => false,
+    canRenameUserChoiceAssociationKey: sinon.stub().returns(false),
+    ...overrides,
+  };
+  sinon.stub(ShellService, "shellService").value(mock);
+  return mock;
+}
+
+registerCleanupFunction(() => {
+  sinon.restore();
+  Services.prefs.clearUserPref(USER_CHOICE_PREF);
+  Services.prefs.clearUserPref(REG_RENAME_PREF);
+});
+
+add_task(function test_pref_disabled() {
+  Services.prefs.setBoolPref(USER_CHOICE_PREF, false);
+  const native = mockNativeShellService({
+    checkAllProgIDsExist: () => {
+      Assert.ok(
+        false,
+        "Should not check capability when the user choice pref is off"
+      );
+      return true;
+    },
+  });
+
+  Assert.strictEqual(
+    ShellService.isOneClickSetDefaultEnabled(),
+    false,
+    "Should be false when the user choice pref is disabled, regardless of capability"
+  );
+  Assert.ok(
+    native.canRenameUserChoiceAssociationKey.notCalled,
+    "Should not probe the registry when the user choice pref is off"
+  );
+
+  sinon.restore();
+});
+
+add_task(function test_user_choice_impossible() {
+  Services.prefs.setBoolPref(USER_CHOICE_PREF, true);
+  const native = mockNativeShellService({
+    checkBrowserUserChoiceHashes: () => false,
+  });
+
+  Assert.strictEqual(
+    ShellService.isOneClickSetDefaultEnabled(),
+    false,
+    "Should be false when the existing UserChoice hashes can't be reproduced"
+  );
+  Assert.ok(
+    native.canRenameUserChoiceAssociationKey.notCalled,
+    "Should not probe the registry when a UserChoice write can't succeed anyway"
+  );
+
+  sinon.restore();
+});
+
+add_task(function test_ucpd_not_running() {
+  Services.prefs.setBoolPref(USER_CHOICE_PREF, true);
+  const native = mockNativeShellService({
+    isUserChoiceProtectionDriverRunning: () => false,
+  });
+
+  Assert.strictEqual(
+    ShellService.isOneClickSetDefaultEnabled(),
+    true,
+    "Should be true when UCPD isn't running to lock the UserChoice keys"
+  );
+  Assert.ok(
+    native.canRenameUserChoiceAssociationKey.notCalled,
+    "Should not probe the registry when UCPD isn't running"
+  );
+
+  sinon.restore();
+});
+
+add_task(function test_already_default_skips_probe() {
+  Services.prefs.setBoolPref(USER_CHOICE_PREF, true);
+  const native = mockNativeShellService({
+    isUserChoiceProtectionDriverRunning: () => true,
+    isDefaultBrowser: () => true,
+  });
+
+  Assert.strictEqual(
+    ShellService.isOneClickSetDefaultEnabled(),
+    true,
+    "Should be true without probing when Firefox is already the default"
+  );
+  Assert.ok(
+    native.canRenameUserChoiceAssociationKey.notCalled,
+    "Should not probe the registry when Firefox is already the default"
+  );
+
+  sinon.restore();
+});
+
+add_task(function test_ucpd_running_rename_blocked() {
+  Services.prefs.setBoolPref(USER_CHOICE_PREF, true);
+  const native = mockNativeShellService({
+    isUserChoiceProtectionDriverRunning: () => true,
+    canRenameUserChoiceAssociationKey: sinon.stub().returns(false),
+  });
+
+  Assert.strictEqual(
+    ShellService.isOneClickSetDefaultEnabled(),
+    false,
+    "Should be false when UCPD is running and blocks the registry rename"
+  );
+  Assert.ok(
+    native.canRenameUserChoiceAssociationKey.calledOnceWith("http"),
+    "Should probe the http association"
+  );
+
+  sinon.restore();
+});
+
+add_task(function test_ucpd_running_rename_permitted() {
+  Services.prefs.setBoolPref(USER_CHOICE_PREF, true);
+  mockNativeShellService({
+    isUserChoiceProtectionDriverRunning: () => true,
+    canRenameUserChoiceAssociationKey: sinon.stub().returns(true),
+  });
+
+  Assert.strictEqual(
+    ShellService.isOneClickSetDefaultEnabled(),
+    true,
+    "Should be true when UCPD is running but permits the registry rename"
+  );
+
+  sinon.restore();
+});
+
+add_task(function test_reg_rename_disabled() {
+  Services.prefs.setBoolPref(USER_CHOICE_PREF, true);
+  Services.prefs.setBoolPref(REG_RENAME_PREF, false);
+  const native = mockNativeShellService({
+    isUserChoiceProtectionDriverRunning: () => true,
+    canRenameUserChoiceAssociationKey: sinon.stub().returns(true),
+  });
+
+  Assert.strictEqual(
+    ShellService.isOneClickSetDefaultEnabled(),
+    false,
+    "Should be false when the rename pref is false, as renaming is the only " +
+      "way to reach a locked UserChoice key"
+  );
+  Assert.ok(
+    native.canRenameUserChoiceAssociationKey.notCalled,
+    "Should not probe the registry when a successful rename wouldn't be used"
+  );
+
+  sinon.restore();
+  Services.prefs.clearUserPref(REG_RENAME_PREF);
+});
diff --git a/src/zen/tests/mochitests/shell/unit/test_taskbarPinningPolicy.js b/src/zen/tests/mochitests/shell/unit/test_taskbarPinningPolicy.js
new file mode 100644
index 000000000..a24d06f28
--- /dev/null
+++ b/src/zen/tests/mochitests/shell/unit/test_taskbarPinningPolicy.js
@@ -0,0 +1,166 @@
+/* Any copyright is dedicated to the Public Domain.
+ * https://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { MockRegistry } = ChromeUtils.importESModule(
+  "resource://testing-common/MockRegistry.sys.mjs"
+);
+
+const EXPLORER_POLICY_KEY = String.raw`Software\Policies\Microsoft\Windows\Explorer`;
+const POLICY_LOG_PREFIX = "Pinning disabled by policy";
+
+const HIVES = {
+  HKCU: Ci.nsIWindowsRegKey.ROOT_KEY_CURRENT_USER,
+  HKLM: Ci.nsIWindowsRegKey.ROOT_KEY_LOCAL_MACHINE,
+};
+const POLICIES = ["NoPinningToTaskbar", "TaskbarNoPinnedList"];
+
+const shellService = Cc["@mozilla.org/browser/shell-service;1"].getService(
+  Ci.nsIWindowsShellService
+);
+
+let gRegistry;
+let gLogFile;
+
+add_setup(async function () {
+  // The pinning policy diagnostic only reaches MOZ_LOG, never the console
+  // service, so route the module to a file and read it back.
+  // logging.config.sync flushes each line, so the message is on disk by the
+  // time canPinToTaskbar() returns.
+  const logBase = PathUtils.join(
+    do_get_profile().path,
+    "taskbar_policy_test.log"
+  );
+  gLogFile = `${logBase}-main.${Services.appinfo.processID}.moz_log`;
+
+  Services.prefs.setBoolPref("logging.config.sync", true);
+  Services.prefs.setCharPref("logging.config.LOG_FILE", logBase);
+  Services.prefs.setIntPref("logging.shell_windows::taskbar", 3);
+
+  gRegistry = new MockRegistry();
+
+  registerCleanupFunction(async () => {
+    gRegistry.shutdown();
+    Services.prefs.clearUserPref("logging.shell_windows::taskbar");
+    Services.prefs.clearUserPref("logging.config.LOG_FILE");
+    Services.prefs.clearUserPref("logging.config.sync");
+    await IOUtils.remove(gLogFile, { ignoreAbsent: true });
+  });
+});
+
+async function readLog() {
+  if (!(await IOUtils.exists(gLogFile))) {
+    return "";
+  }
+  return IOUtils.readUTF8(gLogFile);
+}
+
+function clearPolicies() {
+  for (const root of Object.values(HIVES)) {
+    for (const policy of POLICIES) {
+      gRegistry.setValue(root, EXPLORER_POLICY_KEY, policy, null);
+    }
+  }
+}
+
+function setPolicy(hive, policy, value) {
+  clearPolicies();
+  gRegistry.setValue(
+    HIVES[hive],
+    EXPLORER_POLICY_KEY,
+    policy,
+    value,
+    Ci.nsIWindowsRegKey.TYPE_INT
+  );
+}
+
+// Calls canPinToTaskbar and returns what it threw (if anything) along with the
+// log output it produced. The log file accumulates across tasks, so only the
+// appended portion is returned.
+async function callCanPinToTaskbar() {
+  const offset = (await readLog()).length;
+
+  let error = null;
+  try {
+    shellService.canPinToTaskbar();
+  } catch (e) {
+    error = e;
+  }
+
+  let log = (await readLog()).slice(offset);
+
+  return { error, log };
+}
+
+// Set of hive/group policy combinations known to block pinning.
+const BLOCKING_POLICIES = [
+  ["HKCU", "NoPinningToTaskbar"],
+  ["HKCU", "TaskbarNoPinnedList"],
+  ["HKLM", "TaskbarNoPinnedList"],
+];
+
+add_task(async function test_policiesBlockPinning() {
+  for (const [hive, policy] of BLOCKING_POLICIES) {
+    info(`Testing ${policy} in ${hive}`);
+    setPolicy(hive, policy, 1);
+
+    const { error, log } = await callCanPinToTaskbar();
+
+    Assert.equal(
+      error?.result,
+      Cr.NS_ERROR_NOT_AVAILABLE,
+      `canPinToTaskbar threw NS_ERROR_NOT_AVAILABLE for ${policy} in ${hive}`
+    );
+    Assert.stringContains(
+      log,
+      `${POLICY_LOG_PREFIX} ${policy} in hive ${hive}.`,
+      `The blocking policy ${policy} in ${hive} was logged`
+    );
+  }
+});
+
+// Policies that apply in other hives, but not the ones paired here.
+const INVALID_HIVE_FOR_POLICY = [["HKLM", "NoPinningToTaskbar"]];
+
+add_task(async function test_invalidPoliciesDoNotBlockPinning() {
+  for (const [hive, policy] of INVALID_HIVE_FOR_POLICY) {
+    info(`Testing ${policy} in ${hive}`);
+    setPolicy(hive, policy, 1);
+
+    // Whether pinning is actually available depends on the host machine, so only
+    // the absence of the policy diagnostic is asserted here.
+    const { log } = await callCanPinToTaskbar();
+
+    Assert.ok(
+      !log.includes(POLICY_LOG_PREFIX),
+      `The policy ${policy} in ${hive} is invalid and should not block pinning`
+    );
+  }
+});
+
+add_task(async function test_policySetToZeroDoesNotBlockPinning() {
+  setPolicy("HKCU", "NoPinningToTaskbar", 0);
+
+  // Whether pinning is actually available depends on the host machine, so only
+  // the absence of the policy diagnostic is asserted here.
+  const { log } = await callCanPinToTaskbar();
+
+  Assert.ok(
+    !log.includes(POLICY_LOG_PREFIX),
+    "A policy present but set to zero does not block pinning"
+  );
+});
+
+add_task(async function test_noPoliciesSet() {
+  clearPolicies();
+
+  // Whether pinning is actually available depends on the host machine, so only
+  // the absence of the policy diagnostic is asserted here.
+  const { log } = await callCanPinToTaskbar();
+
+  Assert.ok(
+    !log.includes(POLICY_LOG_PREFIX),
+    "No policy is reported when none are set"
+  );
+});
diff --git a/src/zen/tests/mochitests/shell/unit/xpcshell.toml b/src/zen/tests/mochitests/shell/unit/xpcshell.toml
index 106c8565a..d5f25a0c4 100644
--- a/src/zen/tests/mochitests/shell/unit/xpcshell.toml
+++ b/src/zen/tests/mochitests/shell/unit/xpcshell.toml
@@ -30,6 +30,11 @@ support-files = [
 ]
 tags = ["portal"]
 
+["test_isOneClickSetDefaultEnabled.js"]
+run-if = [
+  "os == 'win'",
+]
+
 ["test_linuxDesktopEntry.js"]
 run-if = [
   "os == 'linux'",
@@ -45,6 +50,11 @@ run-if = [
   "os == 'win'"
 ]
 
+["test_taskbarPinningPolicy.js"]
+run-if = [
+  "os == 'win'",
+]
+
 ["test_writeShortcutIcon.js"]
 run-if = [
   "os == 'win'",
diff --git a/src/zen/tests/mochitests/sync/head_helpers.js b/src/zen/tests/mochitests/sync/head_helpers.js
index d9383db21..17a726501 100644
--- a/src/zen/tests/mochitests/sync/head_helpers.js
+++ b/src/zen/tests/mochitests/sync/head_helpers.js
@@ -97,7 +97,10 @@ function loadWebExtensionTestFunctions() {
   const path = webExtensionsTestPath("/head_sync.js");
   let file = do_get_file(path);
   let uri = Services.io.newFileURI(file);
-  Services.scriptloader.loadSubScript(uri.spec, gGlobalScope);
+  Services.scriptloader.loadSubScriptWithOptions(uri.spec, {
+    target: gGlobalScope,
+    allowUnsafeURL: true,
+  });
 }
 
 /**
diff --git a/src/zen/tests/mochitests/sync/test_resource.js b/src/zen/tests/mochitests/sync/test_resource.js
index 5dee57b39..2e983a655 100644
--- a/src/zen/tests/mochitests/sync/test_resource.js
+++ b/src/zen/tests/mochitests/sync/test_resource.js
@@ -47,10 +47,10 @@ function server_404(metadata, response) {
   response.bodyOutputStream.write(body, body.length);
 }
 
-var pacFetched = false;
+var pacFetched;
 function server_pac(metadata, response) {
   _("Invoked PAC handler.");
-  pacFetched = true;
+  pacFetched.resolve();
   let body = 'function FindProxyForURL(url, host) { return "DIRECT"; }';
   response.setStatusLine(metadata.httpVersion, 200, "OK");
   response.setHeader(
@@ -189,12 +189,13 @@ add_task(async function test_proxy_auth_redirect() {
 
   PACSystemSettings.PACURI = server.baseURI + "/pac2";
   installFakePAC();
+  pacFetched = Promise.withResolvers();
   let res = new Resource(server.baseURI + "/open");
   let result = await res.get();
-  Assert.ok(pacFetched);
+  await pacFetched.promise;
   Assert.ok(fetched);
   Assert.equal("This path exists", result.data);
-  pacFetched = fetched = false;
+  fetched = false;
   uninstallFakePAC();
   await promiseStopServer(server);
 });
diff --git a/src/zen/tests/mochitests/tabMediaIndicator/browser.toml b/src/zen/tests/mochitests/tabMediaIndicator/browser.toml
index a725df4df..ee21b0a23 100644
--- a/src/zen/tests/mochitests/tabMediaIndicator/browser.toml
+++ b/src/zen/tests/mochitests/tabMediaIndicator/browser.toml
@@ -35,16 +35,18 @@ https_first_disabled = true
 
 ["browser_mute.js"]
 
-["browser_mute2.js"]
-
 ["browser_mute_persist_navigation.js"]
 
 ["browser_mute_restore_closed_audible_tab.js"]
 
+["browser_mute2.js"]
+
 ["browser_mute_webAudio.js"]
 
 ["browser_sound_indicator_silent_video.js"]
 
+["browser_sound_indicator_tab_move.js"]
+
 ["browser_webAudio_hideSoundPlayingIcon.js"]
 
 ["browser_webAudio_silentData.js"]
diff --git a/src/zen/tests/mochitests/tabMediaIndicator/browser_mediaplayback_audibility_change.js b/src/zen/tests/mochitests/tabMediaIndicator/browser_mediaplayback_audibility_change.js
index 6c0c9de2b..4c50c2afe 100644
--- a/src/zen/tests/mochitests/tabMediaIndicator/browser_mediaplayback_audibility_change.js
+++ b/src/zen/tests/mochitests/tabMediaIndicator/browser_mediaplayback_audibility_change.js
@@ -179,31 +179,6 @@ add_task(async function testSoundIndicatorWhenChangingMediaVolume() {
 /**
  * Following are helper functions
  */
-function initMediaPlaybackDocument(
-  tab,
-  fileName,
-  { preload, createVideo, muted = false, volume = 1.0 } = {}
-) {
-  return SpecialPowers.spawn(
-    tab.linkedBrowser,
-    [fileName, preload, createVideo, muted, volume],
-    // eslint-disable-next-line no-shadow
-    async (fileName, preload, createVideo, muted, volume) => {
-      if (createVideo) {
-        content.media = content.document.createElement("video");
-      } else {
-        content.media = content.document.createElement("audio");
-      }
-      if (preload) {
-        content.media.preload = preload;
-      }
-      content.media.muted = muted;
-      content.media.volume = volume;
-      content.media.src = fileName;
-    }
-  );
-}
-
 function initMediaStreamPlaybackDocument(tab) {
   return SpecialPowers.spawn(tab.linkedBrowser, [], async _ => {
     content.media = content.document.createElement("audio");
@@ -212,26 +187,6 @@ function initMediaStreamPlaybackDocument(tab) {
   });
 }
 
-function playMedia(tab, { resolveOnTimeupdate } = {}) {
-  return SpecialPowers.spawn(
-    tab.linkedBrowser,
-    [resolveOnTimeupdate],
-    // eslint-disable-next-line no-shadow
-    async resolveOnTimeupdate => {
-      await content.media.play();
-      if (resolveOnTimeupdate) {
-        await new Promise(r => (content.media.ontimeupdate = r));
-      }
-    }
-  );
-}
-
-function pauseMedia(tab) {
-  return SpecialPowers.spawn(tab.linkedBrowser, [], async _ => {
-    content.media.pause();
-  });
-}
-
 function assignNewSourceForAudio(tab, fileName) {
   // eslint-disable-next-line no-shadow
   return SpecialPowers.spawn(tab.linkedBrowser, [fileName], async fileName => {
diff --git a/src/zen/tests/mochitests/tabMediaIndicator/browser_sound_indicator_tab_move.js b/src/zen/tests/mochitests/tabMediaIndicator/browser_sound_indicator_tab_move.js
new file mode 100644
index 000000000..6731d69e4
--- /dev/null
+++ b/src/zen/tests/mochitests/tabMediaIndicator/browser_sound_indicator_tab_move.js
@@ -0,0 +1,76 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Moving a tab to another window hands its browsing context to a different
+ * browser, which also changes the media controller the tab has to listen to.
+ * For each way a tab can reach another window, these tests check that the sound
+ * indicator still appears when audio starts and still clears when audio stops.
+ */
+
+const MOVE_MODES = [
+  { name: "moving it to a new window", moveTab: moveTabToNewWindow },
+  { name: "adopting it into another window", moveTab: adoptTabIntoOtherWindow },
+];
+
+for (const { name, moveTab } of MOVE_MODES) {
+  add_task(async function testSoundIndicatorAppearsAfterMovingTab() {
+    info("create a tab loading media document");
+    const tab = await createBlankForegroundTab();
+    await initMediaPlaybackDocument(tab, "audio.ogg");
+
+    info(`move the silent tab by ${name}`);
+    const { win, movedTab } = await moveTab(tab);
+
+    info("sound indicator should appear when audible audio starts playing");
+    await playMedia(movedTab);
+    await waitForTabSoundIndicatorAppears(movedTab);
+
+    info("stop media and close window");
+    await pauseMedia(movedTab);
+    await BrowserTestUtils.closeWindow(win);
+  });
+
+  add_task(async function testSoundIndicatorDisappearsAfterMovingTab() {
+    info("create a tab loading media document");
+    const tab = await createBlankForegroundTab();
+    await initMediaPlaybackDocument(tab, "audio.ogg");
+
+    info("sound indicator should appear when audible audio starts playing");
+    await playMedia(tab);
+    await waitForTabSoundIndicatorAppears(tab);
+
+    info(`move the audible tab by ${name}`);
+    const { win, movedTab } = await moveTab(tab);
+    ok(movedTab.soundPlaying, "Tab sound indicator is kept after moving");
+
+    info("sound indicator should disappear when audio stops playing");
+    await pauseMedia(movedTab);
+    await waitForTabSoundIndicatorDisappears(movedTab);
+
+    info("close window");
+    await BrowserTestUtils.closeWindow(win);
+  });
+}
+
+// Following are helper functions.
+
+async function moveTabToNewWindow(tab) {
+  const win = gBrowser.replaceTabWithWindow(tab);
+  await TestUtils.topicObserved(
+    "browser-delayed-startup-finished",
+    subject => subject == win
+  );
+  return { win, movedTab: win.gBrowser.selectedTab };
+}
+
+async function adoptTabIntoOtherWindow(tab) {
+  const win = await BrowserTestUtils.openNewBrowserWindow();
+  const movedTab = win.gBrowser.adoptTab(tab, { selectTab: true });
+  if (!movedTab) {
+    throw new Error("adoptTab refused to swap the tab into the other window");
+  }
+  return { win, movedTab };
+}
diff --git a/src/zen/tests/mochitests/tabMediaIndicator/head.js b/src/zen/tests/mochitests/tabMediaIndicator/head.js
index 8d58339a2..ce58e50ee 100644
--- a/src/zen/tests/mochitests/tabMediaIndicator/head.js
+++ b/src/zen/tests/mochitests/tabMediaIndicator/head.js
@@ -12,8 +12,8 @@ const gEMPTY_PAGE_URL = GetTestWebBasedURL("file_empty.html");
  *        if set, then return a url with different origin
  */
 function GetTestWebBasedURL(fileName, cors = false) {
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   const origin = cors ? "http://example.org" : "http://example.com";
   return (
     getRootDirectory(gTestPath).replace("chrome://mochitests/content", origin) +
@@ -163,3 +163,52 @@ function disableNonTestMouse(disable) {
   let utils = window.windowUtils;
   utils.disableNonTestMouseEvents(disable);
 }
+
+/**
+ * Create a media element in the given tab and point it at fileName. The element
+ * is stored on the content window as `content.media` for playMedia/pauseMedia.
+ */
+function initMediaPlaybackDocument(
+  tab,
+  fileName,
+  { preload, createVideo, muted = false, volume = 1.0 } = {}
+) {
+  return SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [fileName, preload, createVideo, muted, volume],
+    // eslint-disable-next-line no-shadow
+    async (fileName, preload, createVideo, muted, volume) => {
+      if (createVideo) {
+        content.media = content.document.createElement("video");
+      } else {
+        content.media = content.document.createElement("audio");
+      }
+      if (preload) {
+        content.media.preload = preload;
+      }
+      content.media.muted = muted;
+      content.media.volume = volume;
+      content.media.src = fileName;
+    }
+  );
+}
+
+function playMedia(tab, { resolveOnTimeupdate } = {}) {
+  return SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [resolveOnTimeupdate],
+    // eslint-disable-next-line no-shadow
+    async resolveOnTimeupdate => {
+      await content.media.play();
+      if (resolveOnTimeupdate) {
+        await new Promise(r => (content.media.ontimeupdate = r));
+      }
+    }
+  );
+}
+
+function pauseMedia(tab) {
+  return SpecialPowers.spawn(tab.linkedBrowser, [], async _ => {
+    content.media.pause();
+  });
+}
diff --git a/src/zen/tests/mochitests/zoom/browser_background_link_zoom_reset.js b/src/zen/tests/mochitests/zoom/browser_background_link_zoom_reset.js
index ac224cc4a..a0692bcf3 100644
--- a/src/zen/tests/mochitests/zoom/browser_background_link_zoom_reset.js
+++ b/src/zen/tests/mochitests/zoom/browser_background_link_zoom_reset.js
@@ -39,7 +39,7 @@ function test() {
   (async function () {
     gTestTab = BrowserTestUtils.addTab(gBrowser);
     await FullZoomHelper.selectTabAndWaitForLocationChange(gTestTab);
-    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    // eslint-disable-next-line sdl/no-insecure-url
     await FullZoomHelper.load(gTestTab, "http://example.org" + TEST_PAGE);
   })().then(testInitialZoom, FullZoomHelper.failAndContinue(finish));
 }
diff --git a/src/zen/tests/mochitests/zoom/browser_background_zoom.js b/src/zen/tests/mochitests/zoom/browser_background_zoom.js
index d757d8e7b..10754e294 100644
--- a/src/zen/tests/mochitests/zoom/browser_background_zoom.js
+++ b/src/zen/tests/mochitests/zoom/browser_background_zoom.js
@@ -1,8 +1,8 @@
 var gTestPage =
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   "http://example.org/browser/browser/base/content/test/zoom/zoom_test.html";
 var gTestImage =
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   "http://example.org/browser/browser/base/content/test/general/moz.png";
 var gTab1, gTab2, gTab3;
 var gLevel;
diff --git a/src/zen/tests/mochitests/zoom/browser_default_zoom_multitab.js b/src/zen/tests/mochitests/zoom/browser_default_zoom_multitab.js
index d20402088..7dea43973 100644
--- a/src/zen/tests/mochitests/zoom/browser_default_zoom_multitab.js
+++ b/src/zen/tests/mochitests/zoom/browser_default_zoom_multitab.js
@@ -4,9 +4,9 @@
 "use strict";
 
 add_task(async function test_multidomain_global_zoom() {
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   const TEST_PAGE_URL_1 = "http://example.com/";
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   const TEST_PAGE_URL_2 = "http://example.org/";
 
   // Prepare the test tabs
@@ -95,9 +95,9 @@ add_task(async function test_multidomain_global_zoom() {
 });
 
 add_task(async function test_site_specific_global_zoom() {
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   const TEST_PAGE_URL_1 = "http://example.net/";
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   const TEST_PAGE_URL_2 = "http://example.net/";
 
   // Prepare the test tabs
diff --git a/src/zen/tests/mochitests/zoom/browser_image_zoom_tabswitch.js b/src/zen/tests/mochitests/zoom/browser_image_zoom_tabswitch.js
index b2357d813..84862abe3 100644
--- a/src/zen/tests/mochitests/zoom/browser_image_zoom_tabswitch.js
+++ b/src/zen/tests/mochitests/zoom/browser_image_zoom_tabswitch.js
@@ -6,7 +6,7 @@
 function test() {
   let tab1, tab2;
   const TEST_IMAGE =
-    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    // eslint-disable-next-line sdl/no-insecure-url
     "http://example.org/browser/browser/base/content/test/general/moz.png";
 
   waitForExplicitFinish();
diff --git a/src/zen/tests/mochitests/zoom/browser_mousewheel_zoom.js b/src/zen/tests/mochitests/zoom/browser_mousewheel_zoom.js
index a814f1dba..8ccbf3457 100644
--- a/src/zen/tests/mochitests/zoom/browser_mousewheel_zoom.js
+++ b/src/zen/tests/mochitests/zoom/browser_mousewheel_zoom.js
@@ -4,7 +4,7 @@
 "use strict";
 
 const TEST_PAGE =
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   "http://example.org/browser/browser/base/content/test/zoom/zoom_test.html";
 
 var gTab1, gTab2, gLevel1;
diff --git a/src/zen/tests/mochitests/zoom/browser_sitespecific_background_pref.js b/src/zen/tests/mochitests/zoom/browser_sitespecific_background_pref.js
index 5756c4d8d..81440fde5 100644
--- a/src/zen/tests/mochitests/zoom/browser_sitespecific_background_pref.js
+++ b/src/zen/tests/mochitests/zoom/browser_sitespecific_background_pref.js
@@ -3,7 +3,7 @@ function test() {
 
   (async function () {
     let testPage =
-      // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+      // eslint-disable-next-line sdl/no-insecure-url
       "http://example.org/browser/browser/base/content/test/zoom/zoom_test.html";
     let tab1 = BrowserTestUtils.addTab(gBrowser);
     await FullZoomHelper.selectTabAndWaitForLocationChange(tab1);
diff --git a/src/zen/tests/mochitests/zoom/browser_sitespecific_video_zoom.js b/src/zen/tests/mochitests/zoom/browser_sitespecific_video_zoom.js
index 94fd0dee5..6f3ebc873 100644
--- a/src/zen/tests/mochitests/zoom/browser_sitespecific_video_zoom.js
+++ b/src/zen/tests/mochitests/zoom/browser_sitespecific_video_zoom.js
@@ -4,10 +4,10 @@
 "use strict";
 
 const TEST_PAGE =
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   "http://example.org/browser/browser/base/content/test/zoom/zoom_test.html";
 const TEST_VIDEO =
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   "http://example.org/browser/browser/base/content/test/general/video.webm";
 
 var gTab1, gTab2, gLevel1;
diff --git a/src/zen/tests/mochitests/zoom/browser_subframe_textzoom.js b/src/zen/tests/mochitests/zoom/browser_subframe_textzoom.js
index e5d40cf58..c456feaea 100644
--- a/src/zen/tests/mochitests/zoom/browser_subframe_textzoom.js
+++ b/src/zen/tests/mochitests/zoom/browser_subframe_textzoom.js
@@ -11,7 +11,7 @@ function test() {
   waitForExplicitFinish();
 
   const TEST_PAGE_URL = 'data:text/html,<body><iframe src=""></iframe></body>';
-  // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+  // eslint-disable-next-line sdl/no-insecure-url
   const TEST_IFRAME_URL = "http://test2.example.org/";
 
   (async function () {
diff --git a/src/zen/tests/mochitests/zoom/browser_tabswitch_zoom_flicker.js b/src/zen/tests/mochitests/zoom/browser_tabswitch_zoom_flicker.js
index df1c3816a..6d5d12981 100644
--- a/src/zen/tests/mochitests/zoom/browser_tabswitch_zoom_flicker.js
+++ b/src/zen/tests/mochitests/zoom/browser_tabswitch_zoom_flicker.js
@@ -10,7 +10,7 @@ function test() {
   Services.prefs.setBoolPref("browser.zoom.siteSpecific", true);
 
   let uri =
-    // eslint-disable-next-line @microsoft/sdl/no-insecure-url
+    // eslint-disable-next-line sdl/no-insecure-url
     "http://example.org/browser/browser/base/content/test/zoom/zoom_test.html";
 
   (async function () {
diff --git a/surfer.json b/surfer.json
index 340cc61c7..f144a9df1 100644
--- a/surfer.json
+++ b/surfer.json
@@ -5,7 +5,7 @@
   "binaryName": "zen",
   "version": {
     "product": "firefox",
-    "version": "154.0.1",
+    "version": "155.0",
     "candidate": "155.0",
     "candidateBuild": 1
   },