Self-Destructing Cookies is an add-on that deletes cookies after they are no longer in use. When I log in to example.com and close the tab, all cookies set by example.com will be deleted (after a configurable time, default 90 seconds I think). It has a whitelist to prevent certain domains (that you want to stay logged into) from having their cookies deleted. It does the same for localStorage and other such technologies.
Very simple, but very effective in preventing tracking.
