For some unknown reasons, Mozilla has removed "Save and Quit" feature from Firefox 4.0 and later versions. In earlier versions, if you opened more than one tabs in Firefox and clicked on Close button, it used to show a dialog box to save your tabs and quit the window.
In Firefox 4.0 and later versions, it no longer allows you to save the tabs. Whenever you click on Close button, it only warns you with a dialog box but doesn't provide any button to save the tabs.
Using this small trick, you can bring back the good old feature in Mozilla Firefox 4.0 and later versions:
1. Open Firefox and type about:config in the addressbar and press Enter. It'll confirm, click on I'll be careful, I promise! button.
2. Now type quitwarning in the Filter box, it'll show following boolean key:
3. The entry "browser.showQuitWarning" would be set to false. Double-click on it or right-click on it and select toggle. It'll set its value to true.
4. Now whenever you close multiple tabs, Firefox will ask you to save the tabs and quit.