As a last resort you could get back to the last "safetypoint before you installed Firefox. It might do the trick.
Not sure what you mean Marcel. I've eventually got back here, as Windows decided to install yet more updates as I went to shut down, then It took ages to re-start!
Windows XP has this thingy where you can return the OS into the previous state before you f**ked up. Helped me many times before. Not sure if it'll work in this case but you'll never know.
Use System Restore to Undo Changes if Problems Occur