As of nightly 20190119095933 the now both display in the left corner of the window. So Firefox now probably supports showing decorations in both corners, now it just needs to be able to show decorations in both corners at once.
Bug 1442082 Comment 2 Edit History
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As of nightly 20190119095933 they now both display in the left corner of the window. So Firefox now probably supports showing decorations in both corners, now it just needs to be able to show decorations in both corners at once.