Firefox 4 no longer asks you if you want to save your tabs whenever you quit the browser. If you want to restore this behaviour, all you need is a little
By default, Firefox is set to open your home page every time you open the browser. It no longer asks you when you quit, however, if you want to save your tabs for next time. Of course, you can still set it to always save your tabs by going to Firefox's Preferences and choosing "Show my windows and tabs from last time" under "When Firefox starts". However, if you want it to prompt you every time like it did in version 3, just type
about:config in your address bar and find the
browser.showQuitWarning preference with the search bar. Double-click it so its value changes to true, then quit your browser. If you've set Firefox to open with your home page or with a blank page, you should see the prompts you to save your tabs every time you quit.
Thanks to dathbe, seiryu and p.observer for pointing this out.