Mozilla Firefox stopped saving your tabs on exit

1. In the address line - type "about:config" (no quotation marks).
2. Click "I'll be careful, I promise!"
3. In the filter field - type browser.showQuitWarning. This filters out all other options
4. Double-click on the only option that is left in that list. Its value switches "true".
5. Close about:config tab
6. Tt should work now!

The above should fix an issue with some recent update to Mozilla Firefox when it stopped even offering to save your tabs on quit. No idea why they introduced this feature update.