nixos-generate-config: Use the Firefox module

The Firefox module installs and enables the selected locale whereas the
Firefox package just uses the default locale. Hence, Firefox's language
was still English even when choosing a language other than English.

nixos/modules/installer/tools/tools.nix

@@ -106,11 +106,12 @@ let
       #   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
       # environment.systemPackages = with pkgs; [