Videos won't play in Firefox, fine in other browsers

I seem to have that bug that was fixed a few updates ago where videos won't play. I have the latest version of Firefox. For example, YouTube will load and the page, progress bar and even subtitles will show, but the video will just have the spinning circle. I have tried videos on other websites (Facebook, Tumblr) and they won't play either.

I've uninstalled/reinstalled Firefox, refreshed Firefox, disabled all add-ons, allowed autoplay of videos/audio but nothing works. The videos all work on Chrome and Edge.