I've noticed the logout problem with Firefox for quite some time, and the curious thing is that only just now when I verified the problem again did I see this message:
Sign out failed!
We could not sign you out because your browser seems to be blocking third party cookies.
- Close all browser windows to sign out.
- To prevent this error in the future, you must enable third party cookies by chaging your browser settings.
Sure enough, I had third party cookies disabled. After enabling them, I'm able to sign out of the Forums using Firefox just fine.
Note: "changing" is misspelled. That's not my typo; it's straight from the LiveID site.