For Firefox 57, the team has been working essentially on improving CPU, GPU and memory usage, as well as concurrency. I know that some people have been looking at energy, but that's not where most efforts were spent.
I hope that our next priority becomes energy. Stay tuned :)
That's great to hear. For a lot of users, Chrome is not the competition, but rather Safari, because Safari is energy efficient and privacy conscious. If Firefox can get as energy efficient, or even just close enough to Safari then it becomes THE clear winner.
Well... new renderer and JS engine are faster and use less memory, which means less CPU cycles are used for the same thing and less information is stored in RAM, there are improvements for Windows and Linux hardware acceleration. This all summed-up means reduction of power consumption.
The new style system is faster, but partly due to parallelizing, which doesn't necessarily mean fewer CPU cycles (though _can_ mean more time spent in CPU sleep state).
I don't know why you think 57 uses less memory than 56 for the "renderer and JS engine". I don't think it does. In fact, the data shows that it actually uses a bit more than 56, though this obviously depends on workload.
(Disclaimer: I work on the Firefox rendering engine and have done some work on the JS engine.)
In theory, race to idle might save it. In practice, my fellow web app devs will throw more crap on the web page until things are slow again and only then push back on feature creep from management.
Note that these kinds of things are typically highly dependent on machine/browser configuration and workload. So that bug's title is overly broad, power consumption is fine for many (most?) people.
