Download Forget Me Not - Forget cookies & other data for Firefox. Make the browser forget website data (like cookies, local storage, etc.), except for the data you want to keep by adding domains to a whitelist, graylist, blacklist, or redlist.
-Never: No cleaning.
-On Startup: Clean only on browser start.
-On Leave: Clean on domain leave and on browser start.
-Instantly: Prevent data from being set, if possible. Otherwise