I've heard of this bug on and off for years now. Clearly it's not important to the Firefox team that Firefox is a viable browser on mac. Every other browser / application seems to be able to work around/with the issues described except them.
I took over a month away from other work I have to do to develop the planeshift crate [1] to solve this exact problem. On the WebRender side, tiling work is advancing quickly as well. OS compositing in WR hasn't landed yet, mostly because work has to be done on the legacy Gecko side to wrangle the Cocoa widgets properly, but it's not true that nobody cares.
Thank you for your dedication on this. Now on the subject of energy use, how about the patch for the following critical macOS bug landing in a Firefox 67 point release?