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: