From MozillaZine Knowledge Base
Firefox 2.0 introduces a built-in Session Restore feature, allowing the user to continue browsing from where they left off if browser restarts. This preference controls when to store extra information about a session: contents of forms, scrollbar positions, cookies, and POST data.
Possible values and their effects
Store extra session data for any site. (Default starting with Firefox 4.)
Store extra session data for unencrypted (non-HTTPS) sites only. (Default before Firefox 4.)
Never store extra session data.
First checked in
Has an effect in