What extensions do you have installed? Have you updated Firefox to the latest version (Help > Check for updates)? Have you tried clearing out all your browser data (Tools > Clear recent history > Everything)? Try downloading CCleaner and make sure the options for Firefox are checked.

The problem as you've described is certainly a problem with your Firefox installation or your computer.

*edit*
That third suggestion, to clear your browser data, might remove stuff you want, so check off what you want it to remove.