Tell me about it. I've had a similar problem since the inception of the boards and it's only gotten worse. Although it's the opposite and posts being tagged as new long after I've read them.
I keep having to update them manually through My Profile, but even then they just return the next day when I call up the site. :p I can have posts showing up as new weeks later and after having 'cancelled' them several times.
No idea if Firefox is the problem, but I do know that I have other compatibility issues between these mbs and Firefox, that I don't have on other mbs I visit. So it might be. If you still have another browser, like OE, you could always go back to that and see if it makes a difference.
LabRat