Try starting Firefox up in Safe Mode and see if the problem still exists. If it doesn't, it's one of your add-ons.
Enable each add-on one by one until the problem arrises. Once it happens again, you've found the culprit! Let me know if that does the job.
I had the same problem and it want away when I uninstalled the 'Ask' toolbar that belied it's name by NOT asking whether I wanted it installed!