It's really that we should be using the OS compositor more than we are, particularly on macOS. I think I'm probably going to be working on this, or at least finding someone to work on it, soon. WebRender should make this easier, because we should be able to get away with using a smaller subset of the OS composition functionality than other browsers do.

Anyway, for now, if you're on macOS, set gfx.compositor.glcontext.opaque to true in about:config for a large increase in energy efficiency at the cost of some visual effects.