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 whether the feature is on.
Possible values and their effects
Record information about the current browser session (open windows and tabs, history, cookies, form data) so it can be restored if necessary. (Default)
Disable the Session Restore service completely.
First checked in
Has an effect in