This belongs in the
feedback forum. You've noticed yourself it's restricted to one part of the forums (actually, it's the same in the iBook forum too, and possibly some others, but I haven't checked), so it's likely not a Firefox issue.
Viewing the source of the page you'll notice:
Normal: font-family: Lucida Grande, Verdana, Geneva, Arial, sans-serif;
PowerBook: font-family: Geneva, Verdana, Arial, sans-serif;
It's the difference between Lucida Grande and geneva you're seeing - not a Firefox doing something wrong. Also, on Windows, you'll see verdana all the time, so you wouldn't notice a difference (I think I'm right in thinking Geneva and Lucida Grande are Mac only fonts...)