You want your browser to update more often than your operating system.
Then why do you base on Fedora and have daily auto updates by default?
I shutdown my laptop every day and update every day. That is fine for me.
Fedora Firefox has some hardening flags that official Firefoxn has not. It is built for Fedora and works really really good.
I did benchmarks some time ago and it is also actually very performant.
Flatpak Firefox does not have the ability to create user namespaces for tab process isolation. This is due to all Flatpaks using the same badness-enumerating seccomp filter, there is no additional hardening possible and they still block userns creation.
Firefox can still isolate tabs via seccomp-bpf but this means it has 1 of its 2 security barriers removed when using a flatpak.
Seeing browsers as an app, it is good to have additional security from the browser to the OS, by sandboxing via flatpak.
But seeing the browser as a platform, passwords, bookmarks, credit card details etc may all be stored in there and a sandbox escape not necessary to steal peoples stuff.
Removing Firefox prevents people from reinstalling it (due to the rpm-ostree bug), and apart from the tarball (which has no desktop integration and is some random binary ran from some random location, likely without SELinux protection (unconfined users)) it is the best browser on Fedora.
installing it with distrobox.
This makes no real sense.
Pro
it can update separately from the OS
it works even with the current rpm-ostree bug
it is the Fedora RPM
it is kinda isolated from the root OS
Con
updates are not automatic and need to be configured
not sure if it has access to user namespace creation because it already runs in a user namespace container
it adds additional boot time and constant RAM usage due to having a container
distrobox does not allow Fedora DNF system upgrades so you need to nuke it and reinstall on a version change (at least every 13 months)
Using the tarball and placing it in /var/usrlocal/bin/ may be better. But still cumbersome.