Why is firefox outside of either "with pkgs"

I’m trying to understand the proper way to edit configuration.nix. Why is firefox outside of either “with pkgs”? thunderbird is in the user section because that makes sense. users.users.chris" = { isNormalUser = true; description = "Christopher"; extraGroups = [ "networkmanager" "wheel" ]; packages = with pkgs; [ thunderbird ]; }; I put the rest in environment.systemPackages = with pkgs; [ so sudo could use them too. environment.systemPackages = with pkgs; [ ...