FF: Reattach expando on every message to Frame

This fixes philc#2830: it stops timesofindia.indiatimes.com from failing due
to FF bug 1408996.

Author: mrmr1993

content_scripts/vimium_frontend.coffee

@@ -221,6 +221,7 @@ Frame =
     @port = chrome.runtime.connect name: "frames"
 
     @port.onMessage.addListener (request) =>
+      root.extend window, root unless extend?
       (@listeners[request.handler] ? this[request.handler]) request
 
     # We disable the content scripts when we lose contact with the background page, or on unload.