Firefox ESR in ascii seems to have problems showing the scrollbar thumb and
checkbox/radio/text input outlines. I've found relevant bug reports on
Firefox bugzilla, but they appear to have been fixed in Firefox 48/49.

https://bugzilla.mozilla.org/show_bug.cgi?id=1230955
https://bugzilla.mozilla.org/show_bug.cgi?id=1234158
https://bugzilla.mozilla.org/show_bug.cgi?id=1262136

Is there any reason why this would be re-appearing in Firefox ESR 52?

Could this issue be what's causing the scrollbars to not show up on
xfce4-terminal, if it's related to gtk3?

-- Tom