Sounds weird. Have you done any internal editing of Firefox, or installed any new add-ons?
If not, it might be some strange virus. Do a virus scan with your scanner, or if you don't have one, download Eset NOD32 Antivirus ( http://www.eset.com ). It's my favorite

.
If nothing comes up, then it might just be a bug with FF. I'm afraid the only way to fix it is to either:
-Fix the source manually
-Reinstall it ( http://forums.cnet.com/5208-6620_102-0.html?forumID=14&threadID=247371&messageID=248461 1 )
-Do a system restore to the last backup (should happen automatically each day, so choose the morning or the night before). Here are instructions on how to restore in Windows XP: http://support.microsoft.com/kb/306084
To find instructions for a different version/OS, google for it: http://www.google.com/search?q=how+to+system+restore