It's so weird to me this type of comment pops up on every firefox discussion, but I use firefox on mac everyday and it's rock solid, never run into any problems.
I think it's for non default scaling.
And the non-standard UI, the weird animations, the increased power consumption, the general non-macOS-ness of it all.
or in Windows
I’ll have to disagree there. It’s by far the best on Windows.
Agreed with you, the best Firefox experience is on Windows. Hardware acceleration is on point, always very snappy rendering with no lag. Can't say the same about FF on macOS or Linux, where the experience is worse.
Support for CoreAnimation compositing was added last year. Support for hardware acceleration on Linux was added this week.