Browser.sessionstore.max tabs undo
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. Since this feature lends itself well to keeping track of data about tabs and windows, it’s also used for re-opening recently closed tabs. This preference controls how many closed tabs are kept track of via the Session Restore service.
Possible values and their effects
A positive integer indicating how many tab closures to remember for later possible restoration. (Default is 10)
First checked in
Has an effect in