> I'm curious (genuine curiosity, not meant sarcastic), why is Firefox only now switching to CoreAnimation? AFAIK CoreAnimation has been around since Leopard which makes it nearly 12 year old, which is positively ancient in the tech world.

I think CA was only moved to the window server relatively recently. Before then, my understanding is that it simply used OpenGL in-process, so it had no energy advantages over Firefox's built-in compositor. There was also a long time when the APIs to host CA content in the window server were private APIs and only Safari could use them.

> Also, coincidentally, do you know what exactly causes the H264 problem on macOS?

I believe, but am not sure, that H.264 is decoded in software in Firefox but is decoded in hardware in Safari.