It's gotta be a choice, right?, since they're enabling or disabling the bounce scroll based on the pref. If it was just system-agnostic the pref wouldn't make a difference.
My understanding is all of the browsers contain a lot of custom widgets. Like, Safari is not drawing text using NSTextView or whatever.
Looking at Firefox and Safari side-by-side they look pretttty similar to me. Some very slight text rendering differences, and Firefox is maybe a touch less smooth to scroll than Safari. The bounce-scroll is a little bit more stiff.
I recorded a video if you want to see what I'm seeing:
https://www.youtube.com/watch?v=-lkAy0O5EDs
There are so many different widget implementations out there now, maybe I'm just losing my sense of what "native" is. But the scroll physics feel right to me, the text looks right, the text selection and input and shortcuts all work...native enough for me!