diff --git a/src/zen/common/modules/ZenStartup.mjs b/src/zen/common/modules/ZenStartup.mjs
index 93e549f56..307fa0dbf 100644
--- a/src/zen/common/modules/ZenStartup.mjs
+++ b/src/zen/common/modules/ZenStartup.mjs
@@ -100,7 +100,16 @@ class ZenStartup {
       delete this.promiseInitializedResolve;
 
       setTimeout(() => {
-        gZenWorkspaces._invalidateBookmarkContainers();
+        // Wait for the natural PlacesToolbar rebuild before invalidating, so
+        // the two async rebuilds don't interleave and duplicate bookmarks.
+        // promiseRebuilt() returns undefined when no rebuild is in flight.
+        const rebuilt =
+          document
+            .getElementById("PlacesToolbar")
+            ?._placesView?.promiseRebuilt() ?? Promise.resolve();
+        rebuilt
+          .catch(console.error)
+          .then(() => gZenWorkspaces._invalidateBookmarkContainers());
       });
     });
   }