How to enable "Save and Quit" feature in Firefox 4.0?

 After downloading the Mozilla Firefox 4.0,Open a new tab in that browser ,and in the address bar type “about:config”.  You see a warning window asking for "be carefull and promise" as under:




Click on the button "I'll be careful, I Promise!" and after this promise to be careful, search for an option called “browser.showQuit Warning“, set it to true, and then check that "Save and Quit" feature is working.