It’s in the Silverblue FAQ: https://docs.fedoraproject.org/en-US/fedora-silverblue/faq/#_how_can_i_play_more_videos_in_firefox_like_youtube
Admittedly, it does say you can use the flathub version.
Fedora Silverblue uses flatpaks for most apps by default. The exception is Firefox because the codes haven’t been sandboxed yet, but they are working on it. If you don’t need to play videos in Firefox, there is a flatpak available.
I realize Firefox is probably the biggest thing that really should be sandboxed, its why I haven’t switched to Silverblue yet.
Interesting, I wonder why Silverblue hasn’t switched yet. Or maybe the documentation is out of date.