After more investigation, it turns out that there are two different bugs.
One in FF 46 (this exact bug) was about an invisible scroll bar, which got fixed in a later version. The other newer bug started in FF 52, which is currently about a very thin scroll bar that regressed in 52 and later (see the screenshots in bug #1675496).
After more investigation, it turns out that there are two different bugs.
One in FF 46 (this exact bug) was about an invisible scroll bar, which got fixed in a later version. The other newer bug started in FF 52, which is currently about a very thin scroll bar that regressed in 52 and later (see the screenshots in bug #1675496).
I have changed the duplicates accordingly.