Firefox for Android will be copying Chrome's scrolling physics for faster scrolling, since that's what Android users are used to. Chrome doesn't use Android's standard scrolling physics, so custom scrolling code is needed to emulate its behavior.
You can test this in Firefox 61 Beta by setting the "apz.android.chrome_fling_physics.enabled" pref to true in about:config. The new behavior will hopefully be enabled by default in Firefox 62.
Ah, I was wondering why my scrolling was getting "fling"ed so much on Nightly. It's kinda unusable because 2/3 of the time, when I lift my thumb off the screen, it gets interpreted as a fling. Maybe I'll get used to being very careful, but for now I'm glad to know there's a toggle.
There are still fling bugs to be fixed. :) Feel free to file a bug in Bugzilla that blocks bug 1448439. Perhaps you are seeing one of these known bugs:
You can test this in Firefox 61 Beta by setting the "apz.android.chrome_fling_physics.enabled" pref to true in about:config. The new behavior will hopefully be enabled by default in Firefox 62.
https://bugzilla.mozilla.org/show_bug.cgi?id=1448439