I have been having similar problems. I am using firefox 3.6, and no matter what sub-forums I visit here, what topics I read, the activity ball icon (or bar) remains saying that there are unread posts, even if I was lat to post in that board. I have Emptied ALL my cookies, history and site preferences to no avail
NINJAEDIT: Just realized this never happened in 3.5... I'll look into this