Added an additional check in stores.js to prevent a strict warning from appearing.

services/sync/modules/stores.js

@@ -168,7 +168,8 @@ SnapshotStore.prototype = {
       delete this._data[oldGUID];
 
       for (let GUID in this._data) {
-        if (this._data[GUID].parentGUID == oldGUID)
+        if (("parentGUID" in this._data[GUID]) &&
+            (this._data[GUID].parentGUID == oldGUID))
           this._data[GUID].parentGUID = newGUID;
       }
     }