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
The mozilla bug is now 'fixed' and landing in nightlies v55, but may continue to affect earlier releases if not brought forward upstream.
before:
after