User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:25.0) Gecko/20100101 Firefox/25.0 (Beta/Release) Build ID: 20131112160018 Steps to reproduce: Use a hidpi mac. Position the FF window, that one part is on a hidpi screen and another part is on lodpi screen. The hidpi-part must be bigger than than the lowdpi-part. The element from where the popup appears must be on lodpi-part. And then let the popup box appear. The screenshot was taken from a "lodpi-screen is right of hidpi screen"-configuration. A similar behavoir can be observed, if the situation (low/hidpi-parts) are vice versa. Actual results: The popup box appeared at the wrong position (see screenshot). It seems, that it is calculated without the hidpi factor. Expected results: The popup box should appear at ther correct position.