Clear the Cache as well, it's under Cookies. Remember to close down and reopen the browser. Give it a few seconds before you reopen the browser. If it still doesn't work than are you able to use IE to attempt to login whilst we work it out. Also have you attempted to change your password? And are you manually typing in your password or letting the firefox auto-fill do it for you?
You can check what passwords, and under what url firefox has saved them by going:
Tools > Options > Privacy > Saved Passwords > View Saved Passwords > Show Passwords
I use Firefox almost exclusively, and I don't have any problems logging-in, I use IE for testing accounts and that's about it.