There isn’t any benefit in using Fedora’s Flatpak other than avoiding an additional runtime, which may not really be an option either unless you really grab every single one of your Flatpak’s from Fedora.
org.freedesktop.Platform is used pretty much everywhere, so trying to avoid it won’t be particularly convenient.
Flathub’s Firefox is actually maintained by Mozilla themselves, whereas the one in Fedora is built and maintained by RedHat and therefore should always be lagging somewhat behind the upstream release.
Unless the Fedora build starts including system integration fixes that for one reason or another don’t get merged into Firefox upstream, there really wouldn’t be any single benefit to using it, not for end users anyway.