I love Firefox, but I've noticed some weird things that happen with UBB:
1 - it seems to have trouble logging out. I'll log out of my board, and it says "logged out," but when I return I'm still logged in.
2 - new message lights are erratic
3 - (most annoying) if there are a lot of graphics on the page, it won't load the page all the way to the bottom until it's finished loading the graphics. On a UBB thread, this means that you don't see the newest posts or the reply box until it has loaded everything else. (It also happens with other non-UBB sites too.)
I don't have these problems on MSIE, and I imported my MSIE settings when I installed it. Has anyone else noticed these things?