nixos-generate-config: Use the Firefox module (#287614)

nixos/modules/installer/tools/tools.nix

···
       #   isNormalUser = true;
       #   extraGroups = [ "wheel" ]; # Enable ‘sudo’ for the user.
       #   packages = with pkgs; [
-      #     firefox
       #     tree
       #   ];
       # };
+
+      # programs.firefox.enable = true;
 
       # List packages installed in system profile. To search, run:
       # \$ nix search wget