Ensure you have about:config pref
widget.disable-native-theme-for-content set to
true (the default in Nightly on linux)
View attached testcase.
See if you can see the scrollbar inside of the scroll track.
Should be able to see scrollbar.
The scrollbar is impossible to see, since it's pretty much the same color as the scroll track.
If you hover/click the scrollbar, or if Firefox loses focus, then the scrollbar color changes and it becomes visible. But when you're just looking at the page & the window is focused, the scrollbar is impossible to see. (It's also still impossible to see as it moves, if you scroll with your mousewheel or keyboard or touchpad.)
I suspect we're doing some sort of "invert-lightness-and-convert-to-gray" trick, to get a scrollbar that we expect to be visible over the page-background; but for sufficiently-neutral shades of gray, this seems to not produce any contrast at all.