I can add comments to a question, if any previous comment is already present. Also, I can add a comment to an answer.
I cannot find any button to add a comment to a question when I'm the first one to comment. Is this a question of permissions, did I just miss it, or is it a bug?
I'm using Firefox 5 and see the same behaviour at Stack Overflow and Exchange, as well as in the previous Firefox version (updated just a few minutes ago).
Update: Right now I can see the "add comment" link in some cases, but on Stack Overflow now I can't even add comments to questions that have been commented on before.