Two years ago I wrote this rant. Firefox had a problem on MacOS on scaled resolutions, resulting in insanely high CPU usage and battery drain.
Issue is finally fixed, and I’m happy to say I’m using it again. The fix is expected to land in the stable version in late October 2019, with the release of Firefox 70.
Meanwhile you can download Firefox Nightly, which is the freshest (and sometimes unstable) version of Firefox.
I’m really happy to be using Firefox again, and I think you should try it as well.