Fixed Firefox popup bug

Author: Foulest

src/main/background.html

@@ -10,6 +10,7 @@
     <script src="util/UrlHelpers.js"></script>
     <script src="util/CacheManager.js"></script>
     <script src="util/Storage.js"></script>
+    <script src="util/MessageType.js"></script>
 
     <!-- Other -->
     <script src="util/other/SmartScreenUtil.js"></script>