With either Internet Explorer or Google Chrome the site appears as it should.
Why is this?
Do you happen to have AdBlocker Plus installed as an add-on with your Firefox? And if so, is it enabled for the site?
The banner ad that appears near the "Who is chatting" area near the top of the page, is considered part of the forum (phpBB software) graphics. If you block that ad, all forum graphics disappear and you're left with an unformatted, plain text on white screen version of the forum.
If you disable ABP for this site, everything comes back.