This is FUD. It definitely is not a “critical” security feature. Firefox flatpak can’t currently do its own internal sandboxing of subprocesses via namespaces, but it does do seccomp bpf filtering. That’s in addition to the standard sandboxing of flatpak itself, which is implemented using namespaces anyways.

If you are extra paranoid, you can tweak the flatpak’s permissions to harden the sandboxing via your distro’s flatpak settings app.