Comment 6 for bug 1762551

After more investigation, it turns out that there are two different bugs.

One in FF 46 (bug #1575896) 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.