I've been lurking here for weeks, but unable to log in and post, since upgrading to Firefox 3. Finally today, hoping SBNation had fixed the problems, I tried again, and encountered a new message, with instructions for FF3 users.
If you're out there unable to log in using FF3: the problem is your third party cookies. Go to Preferences/Privacy, under Cookies press "Exceptions", enter "sbnation.com" and press "Allow".
Extra words to allow posting. I see they've kept some "quaint" features around from the old days.