I came accross one of those annoying sites yesterday that refused to serve pages to me because I was using firefox. (Even more annoying since I didn't happen to notice any activex junk). So I told firefox to spoof Internet Explorer. It appeared to work until firefox came crashing down (that's annoying since I tend to have about 20 browsers/tabs going at once). I was running Session Saver which usually helps in cases like these, but upon re-launch firefox immediately crashed again. And continued to do so upon subsequent launches. So I ran firefox from the command line and saw the following:

\$ firefox

I googled (using konqueror) for "INTERNAL ERROR on Browser End: Expected a version > 5! Version = 4" and came across a mailing list archive that notes the solution (as well as documents a two year old issue). The solution is to find the pref.js ( find ~/.mozilla |grep prefs.js ) file and remove the line containing::
user_pref("general.useragent.override", "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)");