It wouldn’t be my choice, but you can always try installing Firefox as a Flatpak. It’s provided by Mozilla. Media playback works out of the box.
For media outside of your browser, installing the Celluloid flatpak is a great option. It also comes directly from the developer with support for lots of codecs, including the patent-encumbered ones found in .mp4.
I really wanted to avoid installing RPM Fusion in Silverblue, though, because you need to do the uninstall-install dance whenever upgrading to a new Silverblue release.