It's possible you're using an unaccelerated driver. I used to have this problem with my old graphics card which wasn't accelerated under Linux. Then I got a newer PC with a better graphics card which, more importantly supported Linux and the problem vanished.
I've found Firefox to be generally slower than Opera but I've only had that problem under Linux with a graphics card that wasn't supported.
I can't remember it being bad for any other programs, although I remember OpenOffice Draw could be slow when anti-aliasing was turned on and the zoom setting was very high.
I didn't try any 3D games but the manipulating 3D objects under OpenOffice weren't that slow.
Sometimes when you have multiple tabs open and something is waiting for DNS, like ad servers it can slow the entire browser down. *shrug*... lots of variables.
I'm not a fan of Firefox, there are plenty of better browsers, it's just so popular due to successful marketing. It's still better than Internet Explorer but that's not hard to beat.