I was surprised this wasn’t a default behavior anymore. Getting Firefox 4 to save on exit wasn’t in the options either which was odd. This is how you get it back:

  1. Type about:config into the location bar and press enter
  2. Just accept the warning message and you’ll see a huge list of options. Don’t get scared of this.
  3. Type the following into the filter box and make sure each one is set to true. Right click and select “toggle” if it’s false.
  • browser.tabs.warnOnClose
  • browser.warnOnQuit
  • browser.warnOnRestart
  • browser.showQuitWarning