This bug is closed, but regressed by the fix in bug 1630732 when the Mission Control pref 'Displays have separate spaces' is enabled. I suggest bug 1642310 is used to fix that case.
Bug 440895 Comment 30 Edit History
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This bug is closed as resolved in Firefox 75. It was regressed by the fix in bug 1630732 in Firefox 77, if the Mission Control pref 'Displays have separate spaces' is enabled. I suggest bug 1642310 is used to fix that case.