I just tested again and it still doesn't work for me. I am using Firefox 2.0.0.14 on Windows Server 2003 SP2.
Firefox is my main browser and I don't have any problems with it anywhere else. This site is in no special zone and I am not blocking anything, nor do I have anything disabled such as cookies or javascript.
To clarify, when I click "Sign In", the page refreshes, with now empty username and password fields.
I get no JavaScript errors either.
Craig