With latest updates, GNOME Web (Epiphany) can play YouTube videos on the Librem 5, so you won't have to switch to Firefox to do that anymore. And you won't believe where the bug was located[1]! ;) Call `sudo apt install gstreamer1.0-libav` or wait a few days until it's installed by default with the next update and enjoy. @purism
[1] it was in glibc...