This is a workaround to mitigate broken (huge wide) scrollbars in mozilla/Gecko based applications for GTK3 >= 3.20 in versions 52 onwards.
Saying that, I think it now brings those GTK3 scrollbars closer to the Breeze Qt scrollbars in latest releases.
Ref: https://bugs.kde.org/show_bug.cgi?id=377008 and https://bugzilla.mozilla.org/show_bug.cgi?id=1343802
I noticed this as I run ubuntu zesty and use Firefox beta/dev editions, but it does now affect the released v52 onwards.
before:
{F3093537}
after
{F3093579}