Which version of Firefox?? Could you elaborate on what "not properly" means?
I'm running OS X, but for what it's worth, I've been accessing this and another Lithium based forum with SeaMonkey, which uses the same Mozilla Gecko core logic as the FF 3.x series without trouble. Here's my user agent string:
Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.1.16) Gecko/20101123 SeaMonkey/2.0.11
Anyone know what browser features this forum requires? Javascript comes to mind.
Do you have any add-ons which might interfering with forum access?