Oh! I had that same problem.

This was after Firefox ate all of my bookmarks, so I was really frustrated ^_^;; It was driving me nuts! Here's what I did:
Close Firefox down.
For to Start > Programs > Mozilla Firefox
There, it's going to have two options, Firefox and (Safe Mode).
Open Firefox (Safe Mode).
Now close it and open up Firefox like you regularly do.
Just going into safe mode cleared it for me. Hopefully it works for you too. ^_^