Videos won't play in anything but Safari

This is a weird one I haven't seen anyone talking about, and I'm pretty baffled.

I have a Skylake Hack with a GTX 1080, using the official NVidia web driver on Sierra.

A few months ago, I lost the ability to play most videos (YouTube, etc.) in Firefox. Eventually it was all videos in Firefox. Some videos still played in Vivaldi (and I would assume Chrome) until the other day when a new Vivaldi update came out and now no videos play in Firefox, Chrome, or Vivaldi.

Yet videos work just fine in Safari.

Anyone seen this? Any ideas? I can't stand Safari...