They are now because they both use those APIs. You can read about the impact of this specific one here:

https://mozillagfx.wordpress.com/2019/10/22/dramatically-red...

Before using those private APIs, with no other options to reach the same performance as Safari, both Chrome and Firefox were in a completely different league.

It is ridiculous to say that Chrome/Firefox are only popular now because they use private APIs.

Your own article says that Firefox only recently started using them.

CALayer and IOSurface are not private...

CALayerHost and CAContext were listed in the offending apis in this rejection of an electron app https://github.com/electron/electron/issues/20027

Firefox does not use CALayerHost. It uses the public IOSurface and CALayer.contents APIs.

https://mjtsai.com/blog/2019/11/04/electron-apps-rejected-fr...

Are those mentioned in the Mozilla post?

“On macOS” is the important part. Safari is by far the most battery friendly browser there and quite possibly has significant market share too. Mozilla has recently managed to reduce their consumption significantly using these APIs: https://mozillagfx.wordpress.com/2019/10/22/dramatically-red...