The HTTP Logout Firefox add-on allows a Firefox user to log out from all current HTTP Authenticated Sessions (Basic and Digest).

The add-on can be downloaded from https://addons.mozilla.org/en-US/firefox/addon/http-logout/.

If you would like to test the add-on, a sample page that uses basic authentication is available. The add-on is invoked through an option in the Firefox Tools menu... Tools menu
...or the context (right-click) menu. Context (right-click) menu