Firefox 4 does not prompt to save tabs on exit

The solution that worked for me was to set the configuration flag in about:config to:

• > browser.showQuitWarning to true

There are several popular solutions to this issue that did not work for me. Some that I came across while researching this issue:

• > browser.tabs.warnOnClose , see http://kb.mozillazine.org/About%3Aconfig_entries
• > browser.warnOnQuit , see http://kb.mozillazine.org/browser.warnOnQuit
• > browser.warnOnRestart , see http://kb.mozillazine.org/browser.warnOnRestart