diff --git a/src/zen/common/modules/ZenStartup.mjs b/src/zen/common/modules/ZenStartup.mjs
index d64378d3b..31c339d21 100644
--- a/src/zen/common/modules/ZenStartup.mjs
+++ b/src/zen/common/modules/ZenStartup.mjs
@@ -11,6 +11,9 @@ class ZenStartup {
   #hasInitializedLayout = false;
 
   isReady = false;
+  promiseInitialized = new Promise(resolve => {
+    this.promiseInitializedResolve = resolve;
+  });
 
   init() {
     this.openWatermark();
@@ -94,6 +97,8 @@ class ZenStartup {
         .getElementById("tabbrowser-arrowscrollbox")
         .setAttribute("orient", "vertical");
       this.isReady = true;
+      this.promiseInitializedResolve();
+      delete this.promiseInitializedResolve;
     });
   }
 
diff --git a/src/zen/sessionstore/ZenWindowSync.sys.mjs b/src/zen/sessionstore/ZenWindowSync.sys.mjs
index 22a53199f..2b95cf218 100644
--- a/src/zen/sessionstore/ZenWindowSync.sys.mjs
+++ b/src/zen/sessionstore/ZenWindowSync.sys.mjs
@@ -810,11 +810,18 @@ class nsZenWindowSync {
       aOtherTab,
       () => {
         this.log(`Swapping docshells between windows for tab ${aOurTab.id}`);
-        aOurTab.ownerGlobal.gBrowser.swapBrowsersAndCloseOther(
-          aOurTab,
-          aOtherTab,
-          false
-        );
+        try {
+          aOurTab.ownerGlobal.gBrowser.swapBrowsersAndCloseOther(
+            aOurTab,
+            aOtherTab,
+            false
+          );
+        } catch (e) {
+          console.error(
+            `Error swapping browsers for tabs ${aOurTab.id} and ${aOtherTab.id}:`,
+            e
+          );
+        }
 
         // Swap permanent keys
         if (!onClose) {
@@ -1221,9 +1228,15 @@ class nsZenWindowSync {
         {},
         /* zenForceSync = */ true
       );
-      win.gZenWorkspaces.promiseInitialized.then(() => {
-        moveAllTabsToWindow();
-      });
+      win.addEventListener(
+        "MozBeforeInitialXULLayout",
+        () => {
+          win.gZenStartup.promiseInitialized.then(() => {
+            moveAllTabsToWindow();
+          });
+        },
+        { once: true }
+      );
       return;
     }
     moveAllTabsToWindow(true);