Bizzare Login Problem.

[Bacon Butty]

Hopefully this is in the right forum;

Basically, when users login with IE there isn't a problem. However, when logging in with Firefox if they use the top nav bar login and hit enter, it will not work, however, if they click 'login' or 'go' when they enter details, it works fine.

So basically, in Firefox when members use the quick login box at the top and hit enter, it says details are incorrect. But when you click 'go' ('login') button, the user is logged in fine.

I did a quick search but couldn't find anything similar. Any help greatly appreciated.