If it's any consolation, Firefox 3.6.10 broke a number of things, not just in Avast!. I regularly use a site that is powered by Wikia, and could no longer log into that site after the update (other than as guest). I ended up uninstalling 3.6.10 and reinstalling 3.6.8 and my login works again. I don't care what their release notes say, they changed more than they are admitting to!