Firefox is just another browser, like Internet Explorer.
I don't use chat, but I've come to understand that it is sometimes down, due to problems with the site's code.
Try opening a second window of the forum, before you enter chat. There's similar problems when trying to edit a review (getting an error when doing so) but like I mentioned before, you can circumvent that error by opening a second window of the forum, or your own profile page, and then entering chat.
Hope this helped a bit.
JJ