I had a niggling problem with Firefox once, and had to resort to brutal measures. I uninstalled it, and went to:
c:\documents and settings\{user name}\application data (you need to have explorer set to view hidden files & folders)
Then I removed all mention of it there. I think I may have deleted all references to FF in the registry too, but I don't advise that unless you are confident with altering the registry.
After re-installation it ran sweet as a nut.