We need to do this via the Marionette / Firefox UI tests framework, since this involves actually shutting down the browser for the test. The test is as follows: 1) Load a document that uses onbeforeunload to show a dialog to the user that can block unloading unless the user confirms the dialog. 2) Tell the browser to quit 3) Expect the dialog to appear, and then click "Leave Page" to let the quit continue. 4) The browser should quit soon after, without showing any more dialogs See bug 1255511 for further details.
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