--- a/dom/notification/old/NotificationDB.jsm
+++ b/dom/notification/old/NotificationDB.jsm
@@ -70,36 +70,37 @@ var NotificationDB = {
     if (aTopic == "xpcom-shutdown") {
       this._shutdownInProgress = true;
       Services.obs.removeObserver(this, "xpcom-shutdown");
       this.unregisterListeners();
     }
   },
 
   filterNonAppNotifications(notifications) {
+    let result = Object.create(null);
     for (let origin in notifications) {
+      result[origin] = Object.create(null);
       let persistentNotificationCount = 0;
       for (let id in notifications[origin]) {
         if (notifications[origin][id].serviceWorkerRegistrationScope) {
           persistentNotificationCount++;
-        } else {
-          delete notifications[origin][id];
+          result[origin][id] = notifications[origin][id];
         }
       }
       if (persistentNotificationCount == 0) {
         if (DEBUG) {
           debug(
             "Origin " + origin + " is not linked to an app manifest, deleting."
           );
         }
-        delete notifications[origin];
+        delete result[origin];
       }
     }
 
-    return notifications;
+    return result;
   },
 
   // Attempt to read notification file, if it's not there we will create it.
   load() {
     var promise = OS.File.read(NOTIFICATION_STORE_PATH, { encoding: "utf-8" });
     return promise.then(
       data => {
         if (data.length > 0) {