To return to the previous behavior of turning the last closed tab into a blank tab, follow these steps:
- Type about:config into the address bar.
- Accept the warning if it appears by clicking “I’ll be careful, I promise!”
- Find the setting for browser.tabs.closeWindowWithLastTab. You can do this quickly by beginning to type any part of the term into the filter bar.
- Double-click on the browser.tabs.closeWindowWithLastTab preference to change it from true to false.
- Enjoy not closing the window when you close the last tab! (If it doesn’t work immediately, try restarting Firefox.)