What extensions do you have in Firefox? I don't know if that's the problem, but it's usually the placed I would start when I had problems with Firefox when I used it (I use Chrome now).

A good test would be to launch Firefox in safe mode. If you go to Start > Programs > Mozilla Firefox, there should be a shortcut in there for "Mozilla Firefox (Safe Mode)." Don't check anything in the dialog that comes up, or it will make permanent changes. But once the browser launches you should be browsing without any add-ons and stuff.