Markus Stange on the Mozilla Gfx Team Blog:
In Firefox 70 we changed how pixels get to the screen on macOS. This allows us to do less work per frame when only small parts of the screen change. As a result, Firefox 70 drastically reduces the power usage during browsing.
It now uses Core Animation, and it seems to show significant improvements.
The post is also very detailed on what uses Core Animation means, which was almost over my head.