On the Mac, when you have external displays connected the movement of windows/apps between the different displays is controlled through the Window drop down menu, where you will find commands to move the window to a different display. It would be great if Firefox had this functionality like Safari.
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I strongly agree with this idea. My Mac is very inconsistent with putting windows back on the display they were previously on when waking from sleep. Although that is a OS issue, it is much less tedious to move all the windows with "Move to External Display" than if I have to move each manually. "Move to External Display" also seems to restore them to the same location where they were before, instead of having to try to match that by hand.