Type about:config<enter> in the address bar. If a warning screen comes up, press the Be Careful button. At the top of the screen is a search bar. Type browser.tabs.warnOnClose and set it to true.

You can check the startup setting.

• Tools > Options > General > Startup: "When Firefox starts":
  "Show my windows and tabs from last time"
  "Show my home page"
  "Show a blank page"

If you use "Show my windows and tabs from last time" then Firefox won't ask for confirmation when you close Firefox. You can select "Show my home page" or "Show a blank page".

same problem. new computer, 48 version. both setups already existed, including browser.tabs.warnOnClose and set it to true. it was already set to true. Still warns on old computer but not on new> same version both. win10 home both. New one does not have Anniversary version of win10