As explained here: Friend Request Bug
the problem is down to Firefox. This site doesn't really play well with Firefox. We put a fix in the FAQ for Firefox users:
Q: I can't edit posts/use quick reply/edit my friend list! What's up with that?
A: These features do not work properly in the Mozilla Firefox browser. Editing can be done by right clicking the "Edit" button and selecting "open in new tab". To use the other features, you must switch to another browser.