A doorhanger is a term for this sort of warning notification: http://i.imgur.com/huWjWjq.png

As 6a68 mentioned, Firefox presents this doorhanger whenever any site other than addons.mozilla.org attempts to install an add-on.

A 'doorhanger' is this bit of UI: https://www.dropbox.com/s/8yivzcq2gw63pqq/Screen%20Shot%2020...