There is currently a problem with FireFox and the login system on the Control Panel. Currently when you try to log in it will just refresh the control panel if you choose not to allow FireFox to remember your password. However, if you allow FireFox to remember your password then it will let you log in with no problem.
We are aware of the problem and are currently looking to fix it. We hope to release the fix in our upcoming feature/bug release (v1.2).