Per-user Firefox settings should be reset when you delete and start a new Windows profile. But then again maybe when you delete a Windows profile it keeps the folders around, and if you make a new one with the same name it recycles those files. God only knows.

I'd uninstall Firefox, delete C:\DOCUME~1\USERNAME\APPLIC~1\MOZILLA, manually delete the Firefox directory in C:\PROGRA~1, delete anything else you can think of, reinstall Firefox (latest version you can find), and if that doesn't work, take a strong magnet to your harddrive.