Earlier quoted context omitted.
I don't know about Windows or Linux, but on Mac it's far from the best browser. It uses more resources (power/battery, memory, etc) than Safari (and sometimes Chrome[ium]), the UI is a mix of old and new code, and system integration is worse than Safari and Chrome. I use it, but both Chromium and Safari are better on this platform.
It's not that Firefox isn't an excellent browser on its own, but on macOS Safari rises the bar very high if you care about details (most people don't).
I'm a Firefox fanboy, still use it on OSX because it's important to me, but the amount of random pauses, crashes, etc is just painful on OSX. It's like none of the Firefox developers use it on OSX or something.
And before someone says "oh it's your extensions", that IMO leads to two talking points
1. It's 2020 and major extensions still cause memory leaks in Firefox. Debugging them isn't easy at all. Almost all of the resources say something like "Run in safe mode and see if it fixes things" and then lots of posts of "post your about:memory here and I'll analyze it." I'm a software engineer by trade and it's been difficult for me to diagnose...
2. Nah because I don't run any extensions on my OSX firefox install