One would think, but I've seen stranger things happen. It's a quick, easy test, and you'll still end up with the latest, greatest version of Java installed win or lose, so worth a shot. Might seem like a stupid question, but have you already tried clearing the browser cache and cookies?

How about creating a new Firefox profile with no extensions and then trying to login? You can make a new profile by adding "--profilemanager" at the end of the path in the Firefox shortcut, where the 'Target' line is, so it looks like this:

"C:\Program Files\Mozilla\Firefox\firefox.exe" --profilemanager