When I try to start telegram-desktop, I get the following error:
telegram-desktop: error while loading shared libraries: libavcodec.so.58: cannot open shared object file: No such file or directory
https://forum.artixlinux.org/index.php/topic,3582.html
I guess this is also the reason, why Firefox suddenly can't play certain video codecs anymore – right?
R-r-right.
Firefox uses FFmpeg for playing multimedia inside HTML5 <audio> and <video> elements.