I also use Firefox, 3.0.x at the moment. My usual mode of Org surfing is to hit the front Guild page, go to one of my usual sub-forums, then open up all interesting threads in new tabs, go back to the main, hit another sub-forum, open threads in new tabs, go back, etc. Once I've done this for all but the Frontroom and Backroom (which I do separately, too many threads at once usually), I'll read those opened threads. I'm noticing the slowness when I go back to the main Guild page, as well as opening new sub-fora and threads.

Not happening now, maybe it's just a week-day thing.