Download the installation file for the latest version of Mozilla.
Uninstall Mozilla (control panel > add/remove programs)
Install from the installation file
Perform the speed boost here: http://www.freerepublic.com/focus/f-news/1299854/posts
Re-download the addons/apps you use.
If this doesn't work, your problem isn't Firefox. If this hapened to me I'd format the machine and reinstall the OS. Could be a good time to get away from XP and go to Win7.