That could be a firefox bug.
Find your firefox profile directory. There should be a file there labeled "cookies.sqlite". Delete that file (make sure that firefox is not running before you delete it).
The file will be recreated when you next start firefox. However, you will lose all saved cookies.
I have not seen the problem myself, but I have seen reports of it. Apparently the cookies.sqlite file can occasionally become corrupted.
I am currently using firefox 3.6.6 on linux (opensuse 11.3, just installed yesterday).