Since the original issue #18261 was closed without a solution, I created a new one.

Please take a look at this very simple markup using the full screen preview in Firefox: https://www.bootply.com/IBqa3KimQL

At the bottom of the YouTube video is a black line with 1 pixel.
This is especially annoying on a white background page and a white video.
The black line does not show up with Google Chrome or Apple Safari. I have this issue only with Firefox (version 59.0.2 here).