Still Safari is the best in terms of battery endurance (its a VERY SIGNIFICANT difference). But then again its Safari, so you have to decide whether you want a capable browser (FF/Chrome) or a battery efficient one.
This is personal experience, no lab tests, etc.
That seems to be a known issue, but for some reason it's taking a long time to be solved.
Another work around if you want to use a non-default scaled resolution with firefox is to use firefox in non retina mode. I prefer not to use this as it makes the experience blurry.
I do wish they would fix this issue though.
With the handoff working too I’m downloading tonight to give FF a fair shake.
See "Adding support for native OS compositors to WebRender" at https://pcwalton.github.io/2018/12/07/plans-for-2019.html