Hi,
This one was bugging me, because I especially like troubleshooting CSS. Nothing obvious at all! It even seems to mostly validate.
I think I have a lead for you, though. Your sparse DOCTYPE is for HTML5, and I would suggest trying another one... not that FF can't handle HTML5, but who knows what type of small snafu you might have in there somewhere, and that's bleeding edge.
Here they are, a large bounty of them:
http://www.w3.org/QA/2002/04/valid-dtd-list.html
If that's not it, good luck. Maybe somewhere in all that Javascript stuff you have running, and I'm guessing it could be a plugin. But that's a long shot.
Cheers, Dave