I'm wondering if the logoff button actually logs you off, or if it just clears cookies and then you time out. It seems like someone will just sit on a thread for a long time until it says they're logged off.
If it the browser process is actually being ended but the cookies aren't being cleared, you can delete them manually. In Firefox I make all cookies session-only. When the browser gets closed, they get deleted. You can do this with a cookie manager of some kind as well.
It will probably still say you're logged in until you time out.