No problems with YouTube, but almost constant problems with tomplay, on my home built computer with a radeon hd 5450 graphics card. I don't believe that this is a firefox problem, as tomplay plays nicely, both with computers using netbsd and linux, but with different graphics cards.
Here is the xorg log:
https://pastebin.com/Gin1rT0H
Here is some visual documentation:
Linux:

NetBSD (both using the Calm Window Manager):

