Just started happening today for me. I click the "Add Comment" button below any given post and nothing happens. No box appears for me to type, nor do any notifications on closed questions. I'm definitely logged in, have tried logging in and out and I certainly have the required rep for it.
I'm using FireFox 3.6.10. IE 7 & 8 both work fine. Have not tested Safari/Opera/anything else.
Anybody else experiencing this?