no "save Tabs on exit"

One change in Firefox 4 is to by default stop Firefox displaying the warning message, but it can be turned back on by changing some preferences.

1. Type about:config into the location bar and press enter
2. Accept the warning message that appears, you will be taken to a list of preferences  
3. Locate the preference browser.tabs.warnOnClose, if its value is set to false, double-click on it to change its value to true  
4. Repeat this for these 3 preferences browser.warnOnQuit, browser.warnOnRestart, browser.showQuitWarning