Could be a browser problem. FireFox sometimes has incompatibility issues. I was using IE7 and now I'm using IE 8 Beta 2.
I don't recommend upgrading for the end user until the stable release is out though. Definitely, if i works fine on IE then there's a compatibility issue. I don't think I can help much there then. I think primehalo will have to deal with that for you.
EDIT: Just checked your thread with the spoiler tags in it. They work perfectly for me, just like mine do in my forums. I think all sins point towards a FireFox incompatibility.