This is a test case for the Mozilla preference dom.event.contextmenu.enabled. By default, this preference is set to false in order to allow websites to catch calls for the browser's context menu. When this preference is set to true, websites shouldn't be able to disable the browser's context menu (at least, the browser's context menu should appear when you close the alert box). See also: Mozilla Bug #330756, Mozilla Bug #379210, Mozilla Bug #248169