
Actually, its too easy as a,b,c ...
1) First, you need to go to the browser..
2) Second, you need to type in "about:config" at the search column ( apply it without quote )

3) Third, Hit ENTER

4)Search the browser.showQuitWarning.Right click on it, choose toogle
.. Please change it from false to TRUE

5) Make sure after you close it, the firefox will looked like this ..

Happy Hacking Mozilla Firefox with (¯`•-fixcrakerz-.•´¯) (^_^)