The solution is to rebuild Firefox to use Core Animation to draw the window contents and then send only changes to that content to the graphics compositor after that instead of using the long deprecated OpenGL implementation it currently uses that repaints the whole screen over and over, so it's hardly surprising to see such a massive change take quite a while to implement.

The earliest changes in this process have finally made it into beta and there are already substantial improvements in power usage/heat generated by an overworked GPU even before all the changes in progress have been enabled.