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 how often information about the current session is saved to the profile.
Possible values and their effects
A positive integer indicating the number of milliseconds between session saving operations. (Default is 15000, or 15 seconds; in Firefox 3.x: 10 seconds)
First checked in
Has an effect in