I started experiencing the exact same problem a few days ago.
Tried the usual Firefox "clear private data" and CCleaner too.
A few google searches later, found a fix
This is what you do:
go to start menu> search
search and delete the "cookies.sqlite" file
Restart Firefox, do the usual entering password shabang, and voila, everything is fine and dandy again
(this is obviously in my case, if this doesn't work for you.. tough luck lol)