Here is a little more about the add-on from the developers:
Self-Destructing Cookies automatically removes cookies when they are no longer used by open browser tabs. With the cookies, lingering sessions, as well as information used to spy on you, will be expunged. Websites will only be permitted to identify you while you actually use them and can not stalk you across the entire web. This is the closest you will get to cookieless browsing without breaking every second site or tedious micromanaging.
Want to grab the Self-Destructing Cookies add-on? You can pick it up on the Firefox Add-ons website.