None of the Firefox Quantum work has looked at power usage, so don't expect any changes on that front.
"AMD VP9 hardware video decoder support for improved video playback with lower power consumption"
Pretty sure it is the empty set.
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.)