Created attachment 8924146 [details] Screen Shot 2017-11-01 at 14.18.16.png [Affected versions]: Tested on Nightly 58.0a1(2017-10-31), FF release 56 [Affected platforms]: Mac OS X 10.13 [Steps to reproduce]: 1.Open and play a video on youtube or Vimeo or Twitch 2. Enter in fullscreen mode [Expected result]: There are no rendering issues. [Actual result]: In the bottom part of the screen, I see the desktop background. Note: Please see the attachment with the actual result.
This issue has been addressed in 10.13.1.