Rated 5 out of 5 stars
... since FF48.0.1 or so, it cannot change the scrollbar background color anymore (macOSX.8.5). In FF47 and older, this was no problem using the "v2" mode of NewScrollbars.This review is for a previous version of the add-on (188.8.131.52).
You are right, something changed in browsers scrollbar code. I will look into it.