The Google Reader Extension for Firefox has been unavailable since March (our blog post for more information). Per that blog post, we are no longer supporting the extension and don’t know at this time when a new version might be available.
For a number of people upgrading to Firefox 4 meant that they could no longer use the Extension, but for those running older versions, it has still worked.
As of Thursday (May 5th), however, we are shutting down the API that powered those older versions of the Extension, so they will cease to work. To get your PostRank filtering fix, we do recommend the Extensions for Chrome and Safari. (We do not know when a new version may be available, since per the blog post referenced above, the upgrades needed are out of our hands.)