Working version for FF4/5 Rated 5 out of 5 stars
I had the same problem starting with Firefox 5.
The problem arises from the missing "chrome.manifest" - I created one, increased the version to 0.5.5 and set it to be compatible with up to Firefox 5.*:
Original/English version: http://ul.to/7x9atjam
German version: http://ul.to/085cxzjd (thanks to http://www.erweiterungen.de)
THANKS to Jeremy!