[Bug 583797]

[Justdave-mozilla]

Because this wasn't working natively, there's been an extension to do this job for a while now.
https://addons.mozilla.org/en-US/firefox/addon/warn-before-quit/

Said extension is using the legacy extension interface and will no
longer work as of Firefox 57.

The author claims the APIs necessary to make this extension work in
Firefox 57 don't exist (an extension can't tell the browser to quit -
which he has to do after intercepting the original Quit command to put
up the dialog).  https://github.com/nigelbabu/warn-before-
quit/issues/16#issuecomment-322359116

It's either time to get this fixed in core, or help the extension author
find the right API to finish converting it to a WebExtension.

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https://bugs.launchpad.net/bugs/583797

Title:
  "Warn on closing with multiple tabs open" not honored

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