Description of problem: https://github.com/stransky/gecko-dev when youtube video is played in window mode and mouse is over video area (clicking on ads and so) the video playback is flickering.
This bug appears to have been reported against 'rawhide' during the Fedora 27 development cycle. Changing version to '27'.
It looks like this bug has been fixed and is no longer present with Firefox 60.0.1-3 builds on Fedora 28.
I have the problem in firefox-60.0.1-6.fc28.x86_64 when I run firefox-wayland. It happens on other video sites as well.
Fixed upstream - https://bugzilla.mozilla.org/show_bug.cgi?id=1441743