keywords: cannot log out of forum, can't log out, will not log out, won't log out, forum logout fails.
symbolipoint, and anyone else who is having this problem: I was experiencing a similar problem on one computer, but not the other computer, in Mozilla Firefox 3.0.11 and 3.5 (although I don't think the exact version matters). The only way I could log out of the forum was to log out, log back in, then log out twice. After a long time, I finally found a fix for the problem. Try the following.
(1) Close all physicsforums.com (PF) windows, and go to a non-PF web site. Go to your browser menus > Tools > Options > Privacy, and remove your physicsforums.com cookie. (Do this only once, as a permanent fix.) Exit and restart your browser, and see if this resolves the problem. Let us know if this step resolves your problem. If not, try the following step.
(2) Right-click the bookmark(s) or icon(s) you normally use to navigate to the PF forum, and select Properties. If the url Location field begins with "https://www.physicsforums.com/...", change this url prefix to "http://www[/color].physicsforums.com/...". See if this resolves the problem. Let us know if this step resolves your problem.