I have no idea how this happened, but if I click the little icon to display a YouTube video full-screen in Firefox, I get the audio but a black screen. Tried Chrome and it worked fine. So I thought I'd use Chromium, but it has the same problem as Firefox. I've got a fully-upgraded 20.04 installation with Firefox 93.0. Both Chrome and Chromium carry the same version number, Version 94.0.4606.81 (Official Build), though Chromium is a snap.
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