I've been using FF on macOS for the past year and haven't noticed any problems with the battery. I'm not sure what the problems are with scrolling, downloading, or playing media are either to be honest.
Chrome and Safari support keyboard multimedia keys to play/pause video and audio which firefox does not. Picture in Picture is not available on Firefox.
Pinch and double tap gestures to zoom are not available. Scrolling does not follow system conventions like overflow kinetic scrolling.
https://pcwalton.github.io/2018/12/07/plans-for-2019.html mentions about excessive power consumption on webrender.
I find this hugely annoying as well on MacOS when using the touchpad. This is also the reason why I stick to iOS instead of going for Android. All of these details are great on my laptop and phone, though they're not what I really need on my desktop.