A new irefox add-on logs the user out of Facebook when quitting Firefox or after a configurable amount of time has passed since he last closed a Facebook page (and there?s no active tab).

The add-on removes all cookies belonging to facebook.com so even tracking should not be an issue anymore (unless Facebook implements alternative tracking techniques).

If not for privacy issues, the add-on is also useful if someone else uses your computer and wants to post weird status updates.

The add-on is available here