I just retested in case they fixed it and no [1]. The easiest way to tell it's non-native scroll is that Android has this effect when you reach the top or bottom that it "stretches" the content. You can try it everywhere (even inside Firefox settings menu) and it will work like that, except for the actual webpage in Firefox.

Another difference in the scroll is that it has much higher friction (deceleration) in Firefox than native Android.

[1] Firefox 103.2.0 on a Samsung S20 with latest updates.