Running Firefox 3.5.7 and not having any problems at all. I also have the Adblock Plus and the NoScript addons, which stops a lot of the ad crap that tries to break through.

The biggest problem I have is, when responding to post in the forums, the time since my last visit will sometimes change and the new posts since my last visit will change, even if I haven't visited them. I suppose there's some sort of timer involved in that.