Worth noting, this landed a patch that moves some of the UI work to Core Animation on MacOS[1]. More work will happen that builds on top of this, but some reports in that ticket are promising already.
Moving from OpenGL to Core Animation has required a huge amount of work, but even at this early stage (before many of the changes in progress land) the power savings and performance gains are already significant.
[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1429522