From MozillaZine Knowledge Base
This preference controls when cookies expire.
Possible values and their effects
The cookie's lifetime is supplied by the server. (Default)
The user is prompted for the cookie's lifetime.
The cookie expires at the end of the session (when the browser closes).
The cookie lasts for the number of days specified by network.cookie.lifetime.days.
Tools -> Options -> Privacy / Cookies -> Keep Until: (Firefox 2 and above)
Mozilla Suite and SeaMonkey
Edit -> Preferences -> Privacy & Security -> Cookies / Cookie Retention Policy
First checked in
Has an effect in