BleepingComputer.com: Firefox Cookie Problems

I am having trouble with cookies in Firefox on certain forums...when I log in, if I don't choose the "Remember me" option, it logs me out instantly, and I keep having to log in regularly. Even if I do choose "Remember me" or "Keep me logged in", features like "view new posts since last visit", etc. don't work. Anyone know why this is happening and how to fix it? I think it started after I upgraded to Firefox 3 though I can't remember exactly. On another computer of mine, I have a 2.x.x version of Firefox, and these sites work fine, hence my suspicion of Firefox 3. And I know it's Firefox because I don't have any problems in IE for those forums.

Someone suggested deleting the cookies.txt and cookies.sqlite files from my Firefox profile folder, but that didn't help. Someone please suggest something, it's really frustrating! Thanks.

Another user had a similar problem. He reinstalled his Firefox profile after saving his bookmarks and that fixed the problem.
Not sure if he was using Firefox 3 or not. I use Firefox 3 and have not had any problems.