As of Fedora 35, there’s even a filtered list of Flatpaks that can be installed from Flathub without adding Flathub as a remote:
https://fedoraproject.org/wiki/Changes/Filtered_Flathub_Applications
However, none of these are installed by default, as a web browser would have to be. Additionally, Fedora’s Firefox build is included in Workstation too. Also, unless things have changed, Firefox is not part of this list, even though the build of Firefox on Flathub is official and directly comes from Mozilla.
From Fedora’s viewpoint, it still probably makes sense to build something as critical as a browser and have it included by default. You’d want a browser installed “out of the box” so to speak, and it should be Free Software / Open Source.
So… what’s the best approach?
Use GNOME Web as the default and make Firefox easily installable, but not installed by default? GNOME Web is nice, but has some performance issues and has the same issues at playing media that Fedora’s Firefox has and it doesn’t support extensions (yet). Although GNOME Web is a nice, capable browser, for most people it’s more of a sidegrade at best, and many would consider it a downgrade. (Although some people do use it as their main browser.)
Have a snapshot of an official Mozilla Firefox build in a Fedora release, which would be updated once online? It would be a binary blob that’s built by a third party (in Fedora’s point of view) and not built nor controlled by Fedora.
Make the Flathub version of Firefox hide the Fedora one? This seems to make the most sense, so Firefox can be shipped by default and “replaced” by the official Flathub version (even if both might be installed on disk). But it still requires people somehow knowing why Firefox cannot play some videos on the Internet and do something to work around that problem.
Make Fedora’s Firefox compatible with the codecs from Flathub? There are probably technical limitations and UX issues with this approach too. It definitely would require Fedora’s Firefox Flatpak’d instead of being shipped as RPM and also as part of an ostree deplyment.
There’s not a completely straightforward answer.
(For what it’s worth, I install Firefox from Flathub and try to ignore the duplicate on-disk version shipped with Silverblue by pinning the Flathub Flatpak Firefox to my dash in GNOME.)