I would imagine the original post is about Silverblue only, a flatpak is obviously not going to be preinstalled on traditional fedora in the near future.
While it would be nice to have a flatpak, I believe the main reason it’s still in the base image - or at least the only reason I would still be against using the flatpak - is that Firefox is actually less secure in flatpak, as the flatpak sandbox interfers with Firefox’s internal sandboxing. It doesn’t seem to be as badly affected as Chromium-based browsers, but still. Sandboxing Firefox itself can be (is?) done with SELinux.
Is it really way quicker? Fedora gets the latest Firefoxes on a rolling basis as far as I know.
As for Flathub… it would be convenient regarding codecs, but for exactly that and probably other reasons, I doubt we’re going to move away from Fedora flatpaks any time soon.