It varies from person to person... Lately it's not working for me with Firefox, but with IE v?.? it works fine.
One thing I found is on the screen where you enter your username and password -the same as you use to login to the forums- hit the very top login button... IIRC it says "Start FlashChat using built-in login"
Using the other two log in buttons below the name/PW blocks don't work for me.
Get past that and it'll take you to a white? screen where the settings load up, then should go straight to the chat. You may get prompted one more time for a user name/password.
HTH