diff --git a/browser/components/sessionstore/SessionStore.sys.mjs b/browser/components/sessionstore/SessionStore.sys.mjs
index 2c2f43bf743ef458b378e85e9ed44a971711e1d9..1ab8d85dd39a6d5e3def0ee8a2e89bdd1ad3ca35 100644
--- a/browser/components/sessionstore/SessionStore.sys.mjs
+++ b/browser/components/sessionstore/SessionStore.sys.mjs
@@ -127,6 +127,8 @@ const TAB_EVENTS = [
   "TabGroupCollapse",
   "TabGroupExpand",
   "TabSplitViewActivate",
+  "TabAddedToEssentials",
+  "TabRemovedFromEssentials",
 ];
 
 const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
@@ -196,6 +198,7 @@ ChromeUtils.defineESModuleGetters(lazy, {
   TabStateCache: "resource:///modules/sessionstore/TabStateCache.sys.mjs",
   TabStateFlusher: "resource:///modules/sessionstore/TabStateFlusher.sys.mjs",
   setTimeout: "resource://gre/modules/Timer.sys.mjs",
+  ZenSessionStore: "resource:///modules/zen/ZenSessionManager.sys.mjs",
 });
 
 ChromeUtils.defineLazyGetter(lazy, "blankURI", () => {
@@ -1911,6 +1914,8 @@ var SessionStoreInternal = {
       case "TabPinned":
       case "TabUnpinned":
       case "SwapDocShells":
+      case "TabRemovedFromEssentials":
+      case "TabAddedToEssentials":
         this.saveStateDelayed(win);
         break;
       case "TabGroupCreate":
@@ -2020,6 +2025,10 @@ var SessionStoreInternal = {
       this._windows[aWindow.__SSi].isTaskbarTab = true;
     }
 
+    if (aWindow.document.documentElement.hasAttribute("zen-unsynced-window")) {
+      this._windows[aWindow.__SSi].isZenUnsynced = true;
+    }
+
     let tabbrowser = aWindow.gBrowser;
 
     // add tab change listeners to all already existing tabs
@@ -2151,7 +2160,6 @@ var SessionStoreInternal = {
       if (closedWindowState) {
         let newWindowState;
         if (
-          AppConstants.platform == "macosx" ||
           !lazy.SessionStartup.willRestore()
         ) {
           // We want to split the window up into pinned tabs and unpinned tabs.
@@ -2215,6 +2223,15 @@ var SessionStoreInternal = {
       });
       this._shouldRestoreLastSession = false;
     }
+    else if (!aInitialState && isRegularWindow) {
+      let windowPromises = [];
+      for (let window of this._browserWindows) {
+        windowPromises.push(lazy.TabStateFlusher.flushWindow(window));
+      }
+      Promise.all(windowPromises).finally(() => {
+        lazy.ZenSessionStore.restoreNewWindow(aWindow, this);
+      });
+    }
 
     if (this._restoreLastWindow && aWindow.toolbar.visible) {
       // always reset (if not a popup window)
@@ -2465,7 +2482,7 @@ var SessionStoreInternal = {
       // 2) Flush the window.
       // 3) When the flush is complete, revisit our decision to store the window
       //    in _closedWindows, and add/remove as necessary.
-      if (!winData.isPrivate && !winData.isTaskbarTab) {
+      if (!winData.isPrivate && !winData.isTaskbarTab && !winData.isZenUnsynced) {
         this.maybeSaveClosedWindow(winData, isLastWindow);
       }
 
@@ -2486,7 +2503,7 @@ var SessionStoreInternal = {
 
         // Save non-private windows if they have at
         // least one saveable tab or are the last window.
-        if (!winData.isPrivate && !winData.isTaskbarTab) {
+        if (!winData.isPrivate && !winData.isTaskbarTab && !winData.isZenUnsynced) {
           this.maybeSaveClosedWindow(winData, isLastWindow);
 
           if (!isLastWindow && winData.closedId > -1) {
@@ -2582,6 +2599,7 @@ var SessionStoreInternal = {
       let alreadyStored = winIndex != -1;
       // If sidebar command is truthy, i.e. sidebar is open, store sidebar settings
       let shouldStore = hasSaveableTabs || isLastWindow;
+      lazy.ZenSessionStore.maybeSaveClosedWindow(winData, isLastWindow);
 
       if (shouldStore && !alreadyStored) {
         let index = this._closedWindows.findIndex(win => {
@@ -3373,7 +3391,7 @@ var SessionStoreInternal = {
     if (!isPrivateWindow && tabState.isPrivate) {
       return;
     }
-    if (aTab == aWindow.FirefoxViewHandler.tab) {
+    if (aTab == aWindow.FirefoxViewHandler.tab || aTab.hasAttribute("zen-empty-tab")) {
       return;
     }
 
@@ -4089,6 +4107,12 @@ var SessionStoreInternal = {
         Math.min(tabState.index, tabState.entries.length)
       );
       tabState.pinned = false;
+      tabState.zenEssential = false;
+      tabState.zenSyncId = null;
+      tabState.zenIsGlance = false;
+      tabState.zenGlanceId = null;
+      tabState.zenHasStaticLabel = false;
+      tabState.zenWorkspace = aWindow.gZenWorkspaces.activeWorkspace;
 
       if (inBackground === false) {
         aWindow.gBrowser.selectedTab = newTab;
@@ -4525,6 +4549,7 @@ var SessionStoreInternal = {
       // Append the tab if we're opening into a different window,
       tabIndex: aSource == aTargetWindow ? pos : Infinity,
       pinned: state.pinned,
+      essential: state.zenEssential,
       userContextId: state.userContextId,
       skipLoad: true,
       preferredRemoteType,
@@ -5374,7 +5399,7 @@ var SessionStoreInternal = {
 
     for (let i = tabbrowser.pinnedTabCount; i < tabbrowser.tabs.length; i++) {
       let tab = tabbrowser.tabs[i];
-      if (homePages.includes(tab.linkedBrowser.currentURI.spec)) {
+      if (homePages.includes(tab.linkedBrowser.currentURI.spec) && !tab.hasAttribute("zen-empty-tab")) {
         removableTabs.push(tab);
       }
     }
@@ -5483,7 +5508,7 @@ var SessionStoreInternal = {
 
     // collect the data for all windows
     for (ix in this._windows) {
-      if (this._windows[ix]._restoring || this._windows[ix].isTaskbarTab) {
+      if (this._windows[ix]._restoring || this._windows[ix].isTaskbarTab || this._windows[ix].isZenUnsynced) {
         // window data is still in _statesToRestore
         continue;
       }
@@ -5625,11 +5650,16 @@ var SessionStoreInternal = {
     }
 
     let tabbrowser = aWindow.gBrowser;
-    let tabs = tabbrowser.tabs;
+    let tabs = aWindow.gZenWorkspaces.allStoredTabs;
     /** @type {WindowStateData} */
     let winData = this._windows[aWindow.__SSi];
     let tabsData = (winData.tabs = []);
 
+    winData.activeZenSpace = aWindow.gZenWorkspaces?.activeWorkspace || null;
+    winData.splitViewData = aWindow.gZenViewSplitter?.storeDataForSessionStore();
+    winData.folders = aWindow.gZenFolders?.storeDataForSessionStore() || [];
+    winData.spaces = aWindow.gZenWorkspaces?.getWorkspaces();
+
     // update the internal state data for this window
     for (let tab of tabs) {
       if (tab == aWindow.FirefoxViewHandler.tab) {
@@ -5652,7 +5682,7 @@ var SessionStoreInternal = {
     // a window is closed, point to the first item in the tab strip instead (it will never be the Firefox View tab,
     // since it's only inserted into the tab strip after it's selected).
     if (aWindow.FirefoxViewHandler.tab?.selected) {
-      selectedIndex = 1;
+      selectedIndex = 0;
       winData.title = tabbrowser.tabs[0].label;
     }
     winData.selected = selectedIndex;
@@ -5765,8 +5795,8 @@ var SessionStoreInternal = {
     // selectTab represents.
     let selectTab = 0;
     if (overwriteTabs) {
-      selectTab = parseInt(winData.selected || 1, 10);
-      selectTab = Math.max(selectTab, 1);
+      selectTab = parseInt(winData.selected || 0, 10);
+      selectTab = Math.max(selectTab, 0);
       selectTab = Math.min(selectTab, winData.tabs.length);
     }
 
@@ -5821,6 +5851,9 @@ var SessionStoreInternal = {
         savedTabGroup => !openTabGroupIdsInWindow.has(savedTabGroup.id)
       );
     }
+    aWindow.gZenFolders?.restoreDataFromSessionStore(winData.folders);
+    aWindow.gZenViewSplitter?.restoreDataFromSessionStore(winData.splitViewData);
+    aWindow.gZenWorkspaces?.restoreWorkspacesFromSessionStore(winData);
 
     // Move the originally open tabs to the end.
     if (initialTabs) {
@@ -6372,6 +6405,25 @@ var SessionStoreInternal = {
 
     // Most of tabData has been restored, now continue with restoring
     // attributes that may trigger external events.
+    if (tabData.zenEssential) {
+      tab.setAttribute("zen-essential", "true");
+      tabData.pinned = true; // Essential tabs are always pinned.
+    }
+    if (tabData.zenIsEmpty) {
+      tab.setAttribute("zen-empty-tab", "true");
+    }
+    if (tabData.zenHasStaticLabel) {
+      tab.setAttribute("zen-has-static-label", "true");
+    }
+    if (tabData.zenSyncId) {
+      tab.setAttribute("id", tabData.zenSyncId);
+    }
+    if (tabData.zenDefaultUserContextId) {
+      tab.setAttribute("zenDefaultUserContextId", true);
+    }
+    if (tabData.zenWorkspace) {
+      tab.setAttribute("zen-workspace-id", tabData.zenWorkspace);
+    }
 
     if (tabData.pinned) {
       tabbrowser.pinTab(tab);
@@ -7290,7 +7342,7 @@ var SessionStoreInternal = {
 
       let groupsToSave = new Map();
       for (let tIndex = 0; tIndex < window.tabs.length; ) {
-        if (window.tabs[tIndex].pinned) {
+        if (window.tabs[tIndex].pinned && false) {
           // Adjust window.selected
           if (tIndex + 1 < window.selected) {
             window.selected -= 1;
@@ -7305,7 +7357,7 @@ var SessionStoreInternal = {
           );
           // We don't want to increment tIndex here.
           continue;
-        } else if (window.tabs[tIndex].groupId) {
+        } else if (window.tabs[tIndex].groupId && false) {
           // Convert any open groups into saved groups.
           let groupStateToSave = window.groups.find(
             groupState => groupState.id == window.tabs[tIndex].groupId