Firefox 4, released today, does not seem to ask to save open tabs when shutting down. It always asked that in version 3.x, so I consider this a regression. To restore the previous behavior:
- Navigate to about:config and accept the warning
- type quit in the filter box
- Make sure that the browser.showQuitWarning key is set to true.
It’s surprising how Mozilla slipped on this seemingly trivial setting, which should be enabled by default. On a related note, very much like in Google Chrome, you can now pin tabs as Apps (right click on the tab, Pin as App Tab).