Its even worse when you force Firefox to use wayland its icon doesn't even show.
Edit: Oh since everyone now is confused; I only have the flatpak version of Firefox installed yet it doesn't use the pinned icon and doesn't even use the firefox icon under wayland at all.
Regarding docker/podman that's why I wrote depending on usecase, for servers it makes sense to distribute because of scalability, on a single user OS it does not.
From what you write I guess that nix does the distribution part of flatpak, so that seems fine, there's probably a catch/limitation somewhere, there usually is, but it could be an acceptable one.