enable that “fedora-cisco-openh264” dnf repo. (actually, you can do that from inside GNOME Software → software sources) and it seems to work fine. (i.e. rom-ostree shows me it uses this repo when downloading updates)
This way, you need no (inofficial) rpmfusion repo (the Cisco repo is quite official and maintained by Fedora AFAIK, it only has legal reasons as you can read on the wiki).
In the end I also saw the H264 plugin in the rpm-ostree-Firefox and even in the Fedora flatpaks one (from registry.fedoraproject.org), but in both cases youtube.com/html5 claimed H.264 was not available.
It’s probably just not available to the flatpack environments. I’d try on a
normal system and see if that works. If so, you know it’s flatpack which is
broken.
As I’ve mentioned I’ve tried it (even) with the rpm-ostree (i.e. build-in version) of Firefox in Silverblue. I would not even have expected it to appear in the flatpak version. But both don’t work.
Rpmfusion is developed in tandem with Fedora and afaik is the closest to official a proprietary repo can get. It’s highly recommended overall.
I believe the Cisco h264 repos only cover WebRTC h264 for legal reasons. You need rpmfusion and the compat-ffmpeg28 package to be able to use them on Firefox for standard video.
The easiest way for me was to install Google Chrome which includes H.264.
The problem with Chrome in Silverblue is that you cannot update it from the Software Center. You click download and install, reboot and the update is still available. The workaround is to uninstall it, reboot and then install the new version.
The GOOD news is that Fedora 31 will include OpenH264 2.0 by default and it will support the Main and High profiles. It is mentioned at the end of the blog post.
I hope that it will be back-ported to Fedora 30 so we won’t need to install Chrome only for HBO Go or Netflix.
Okay, great, I’ve tested it and installing compat-ffmpeg28 really fixes it (for the rpm-ostree Firefox! Flatpak(ed) Firefox (obviously) has not access to it.
What I wanted to add: GNOME Software works perfectly with rpm-ostree and thus you do not have to use the terminal to add the RPM Fusion free repo, but can now (in Silverblue 30) just download the file and open it with GNOME Software.