rearrange firefox and vpn modules

modules/firefox/extensions.nix

@@ -0,0 +1,32 @@
+args:
+
+{
+  packages = let
+    firefox-addons = args.firefox-addons or null;
+    pkgs = args.pkgs;
+    system = pkgs.stdenv.hostPlatform.system;
+
+    buildFirefoxXpiAddon = firefox-addons.lib.${system}.buildFirefoxXpiAddon or null;
+
+    zotero-connector = if buildFirefoxXpiAddon != null then
+      buildFirefoxXpiAddon {
+        pname = "zotero-connector";
+        version = "5.0.195";
+        addonId = "zotero@chnm.gmu.edu";
+        url = "https://download.zotero.org/connector/firefox/release/Zotero_Connector-5.0.195.xpi";
+        sha256 = "gTfwxRyzJ3e92+bvvt52eXUE2mhRhPybq1gqKAdtwcg=";
+        meta = {};
+      }
+    else null;
+
+  in
+    (if firefox-addons != null then
+      with firefox-addons.packages.${system}; [
+        ublock-origin
+        vimium
+        darkreader
+        cookies-txt
+      ]
+    else [])
+    ++ (if zotero-connector != null then [ zotero-connector ] else []);
+}