feat(browsers): add firefox

- add nur
- set firefox as main browser
- move browsers into separate modules

system/default.nix

@@ -5,6 +5,7 @@
     ./hardened.nix
     ./i18n.nix
     ./nix.nix
+    ./nur.nix
     ./vpn.nix
   ];
 

system/nur.nix

@@ -0,0 +1,9 @@
+{ config, pkgs, ... }:
+
+{
+  nixpkgs.config.packageOverrides = pkgs: {
+    nur = import (builtins.fetchTarball "https://github.com/nix-community/NUR/archive/master.tar.gz") {
+      inherit pkgs;
+    };
+  };
+}