As someone who has worked a bit on energy usage: it's a really, really hard problem, and the tools used to develop/optimize for energy are really, really bad.

Also, being cross-platform complicates things a lot: I'm certain that Safari has access to a number of libraries that are energy-efficient but exist only on macOS/iOS.

I have a vague hope that maybe the Rust community can find a way to somehow develop a standard benchmarking tool that takes into account energy use. This would help considerably on the front of refactoring Firefox towards better energy efficiency.