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.
From what I gather nix is more of a next generation package manager than a application container/sandbox which means potential security problems with old libs could be less, or rather they are probably at the same level as rpm/deb.
I don't see any problems with rpm/deb/etc. ending up getting the boot by nix or another package manager just because they are better, that's just evolution.
As someone said about flatpak/snap that their 'hidden' strength is distribution of proprietary software, that's fine by me if that's the main usage of them.
The sandbox feature can be solved by SELinux/docker/and several other ways depending on usecase.