What do I do about "No video with supported format and MIME type found"?

I run Devuan GNU/Linux Excalibur (i.e. Debian trixie without systemd), with Firefox v123. Recently, I've been getting errors trying to browse content involving videos (yes, from the accursed social media sites, e.g. twitter) in browser windows. I get a gray area and the message: "No video with supported format and MIME type found". Now, first, I'm not even told what MIME type + format combinations _were_ found; how can I figure that out? And second - how can I stack up on more MIME+format combination support?