Chrome and Safari support keyboard multimedia keys to play/pause video and audio which firefox does not. Picture in Picture is not available on Firefox.
Pinch and double tap gestures to zoom are not available. Scrolling does not follow system conventions like overflow kinetic scrolling.
https://pcwalton.github.io/2018/12/07/plans-for-2019.html mentions about excessive power consumption on webrender.
I find this hugely annoying as well on MacOS when using the touchpad. This is also the reason why I stick to iOS instead of going for Android. All of these details are great on my laptop and phone, though they're not what I really need on my desktop.
Until recently Firefox's URL bar had a THICK blue glow when it was in focus, instead of the more subtle one the rest of Mac OS has. It infuriated me. They didn't fix it for years. Why? Because nobody noticed it or cared enough, I guess.
"They didn't fix it" implies someone reported it as a bug. Even if no one did, if you were infuriated by it, surely you did, right?