Let me be clear: I like Firefox. I've committed patches to Firefox on iOS. I'd use Firefox on any other platform, but it is not a good citizen of macOS. I don't believe it'd be good on iOS given the room to use its own rendering engine.
I don’t think any third-party browser can hope to compete with Safari on macOS, though. I don’t know how level the playing field is there.
The bug in question: https://bugzilla.mozilla.org/show_bug.cgi?id=1429522
A blog post breakdown: https://mozillagfx.wordpress.com/2019/10/22/dramatically-red...
This does make a difference, but macOS is in many respects a "death by a thousand cuts" platform due to the UI/UX bar being fairly high. It notably clips in many areas where Chrome and Safari simply don't, which leads to a jarring experience. Scrolling and resizing is still not smooth, even after tweaking it behind the scenes - although to be fair, the post above calls out that they're aware they can make this smoother with CA.
Firefox on macOS also still has issues with hardware acceleration. Comparing the following URL across browsers I can achieve 60fps, no drops, on Safari. Firefox on my Mac averages 45fps.
https://webglsamples.org/aquarium/aquarium.html
I love Firefox (and the associated lineage) - I really do. I've kept it installed since the Mozilla Suite days. I still check every damn release to see if I could switch back. I'm sad every time I find I can't.