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:

* Weirdness at end of Fennec fling. Don't send duplicate DOM 'scroll' event for sub-pixel scrolling: https://bugzilla.mozilla.org/show_bug.cgi?id=1230176

* Scrolling in Android has a distinct hitch when releasing finger: https://bugzilla.mozilla.org/show_bug.cgi?id=1425739

You can also tweak the fling physics parameters with these about:config prefs:

  apz.android.chrome_fling_physics.friction
  apz.android.chrome_fling_physics.inflexion
  apz.android.chrome_fling_physics.stop_threshold