No Cookies For Old Men от em_te

How it works:

This extension listens for clicks and keyboard typing on webpages (which indicate that the webpage is still active) and adds it's domain to a List and sets a counter (default = 2). The domain and its counter is removed from the List after 30 seconds.

This extension also listens for cookie changes (including the adding of new cookies), and also checks if the domain of the cookie is in the List of active domains. If no match found, it overwrites the cookie with a new one with no expiration date thereby turning it into a session cookie. If the domain matches an active domain and the counter for that domain is not zero, it then checks the expiration date of the cookie and if it is longer than 1 week then it overwrites the cookie with a 1 week expiration date and then decrements the active domain's counter.