Silverblue has Firefox installed as an RPM.
There are multiple ways to add the missing codecs, the easiest one is to add the rpmfusion RPMs, reboot and install libavcodec-freeworld
.
So I suppose you have the Flatpak AND the RPM installed. I would recommend the Fedora RPM, it works great and is really fast.
Also, you get more answers on discussion.fedoraproject.org
The Signal one will just be a metadata issue.
flatpak update -y
kill -9 gnome-software
pkcon refresh