Firefox usually asks whether you want to save your tabs on exit if you have browser.showQuitWarning = true in your about:config. If you check "Do not ask next time" it will not ask you again. The question is how to re-enable this prompt. You already set the following:
browser.showQuitWarning = true browser.tabs.warnOnClose = true browser.tabs.warnOnCloseOtherTabs = true browser.warnOnQuit = true
But it still doesn't show the prompt. The corresponding setting is hidden behind browser.startup.page. Just locate it in about:config, right click on it and select "Reset". Restart Firefox and viola, the prompt is back. You can also re-enable this through the UI, go to "Edit -> Preferences -> General -> When Firefox starts" and change "Show my windows and tabs from last time" to "Show my home page" or "Show a blank page". A bit illogical on the first sight, but it works. This was tested on Firefox 31.0.
