I don't like encouraging mucking around in about:config, but I might as well mention this one. If you don't mind your window losing vibrancy and rounded corners, you should be able to significantly improve battery life on macOS by setting "gfx.compositor.glcontext.opaque" to true. This makes WindowServer stop drawing whatever is behind the Firefox window.
This bug tracks the proper solution: https://bugzilla.mozilla.org/show_bug.cgi?id=1429522
Sorry to hijack this thread, but I noticed in your presentation (can't find it now) you could test out pathfinder with --features=pathfinder on Firefox Nightly. Any way to tell if this really enables pathfinder? I ask because I can't tell a difference on Windows or Mac (which may mean it's working as intended).