From MozillaZine Knowledge Base
Firefox 2 introduces a built-in Session Restore feature, allowing the user to continue browsing from where they left off if browser restarts. This preference used to control whether the last saved session is restored each time the browser starts.
It has been superseded by browser.startup.page in builds with bug 340898 fixed (Firefox 2 beta 2 and later, or nightly builds after July 19, 2006).
Possible values and their effects
Restore the previous session each time the browser starts.
Do not restore the previous session. (Default)
First checked in
Has an effect in