Make sure that you do not run Firefox in permanent Private Browsing mode (Never remember history).

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use Private Browsing mode"

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• "3-bar" menu button > Exit (Power button)
• Windows: File > Exit
• Mac: Firefox > Quit Firefox
• Linux: File > Quit

Firefox uses the sessionstore.js file in the profile folder to store session data, but this file is only present when Firefox is closed. The sessionstore.js file is created from recovery.js when you close Firefox and is removed and copied to previous.js when you start Firefox to make is possible to restore the session at any time.

You can try to make a backup copy of the recovery.js just before you want to close Firefox and place this file as sessionstore.js in the main profile folder to see if that at least makes Firefox restore the previous session.

When Firefox crashes it does not create a sessionstore.js file since obviously it probably can't due the crash. Likewise ~\sessionstore_backup\previous.js is not created when Firefox crashes. So if you rarely close Firefox with Power Button, these files don't exist in the event of a crash. Nothing to do but manually copy session files periodically, but that gets old and easy to forget. Thanks.

So the scenario is recovery after a Firefox crash, not recovery after orderly shutdown of Firefox. To recover from a crash, the recovery.js or sessionstore.js should be written to and saved periodically during a session by Firefox.