See this thread for more info: http://www.thefiringline.com/forums/...d.php?t=337650
This should be a one time thing. The best course of action is to hit the "Mark Forums Read" (at the bottom of the forums list or in the Quick Links). Do it after you have read most of what you want to read now if you wish. From then on, the new posts will be reflected properly. The "unviewed" thread marker will now remain on until you actually read the new posts (or 10 days time whichever comes first).