I'm using Firefox, and tonight, though I was logged in, when I tried to post a comment to a video, I'd get a page reminding me to log in. Happened several times.
Same exact thing, using IE, and the comment posted fine. So I'm letting you know that something doesn't seem to be behaving well in Firefox.

(Seems to me it's worked find on video comments before--- I don't comment on every video, but don't recall having this trouble before. I've had no trouble logging in to or posting in the forums.