| 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 2 preferences browser.warnOnQuit, browser.warnOnRestart
5. Starting with beta 12 there will be an additional preference that needs to be set to true - browser.showQuitWarning |