firefox: add support for add-on packages

Since no official source of packages exist the option is hidden for
now. For adventurous people there is an overlay of a few selected
add-ons available at

    https://rycee.gitlab.io/nixpkgs-firefox-addons/overlay.tar.gz

This overlay is automatically built daily using the REST API available
on https://addons.mozilla.org/.

modules/programs/firefox.nix

@@ -6,6 +6,8 @@ let
 
   cfg = config.programs.firefox;
 
+  extensionPath = "extensions/{ec8030f7-c20a-464f-9b0e-13a3a9e97384}";
+
 in
 
 {
@@ -22,6 +24,23 @@ in
         description = "The unwrapped Firefox package to use.";
       };
 
+      extensions = mkOption {
+        type = types.listOf types.package;
+        default = [];
+        example = literalExample ''
+          with pkgs.firefox-addons; [
+            https-everywhere
+            privacy-badger
+          ]
+        '';
+        description = ''
+          List of Firefox add-on packages to install. Note, it is
+          necessary to manually enable these extensions inside Firefox
+          after the first installation.
+        '';
+        visible = false;
+      };
+
       enableAdobeFlash = mkOption {
         type = types.bool;
         default = false;
@@ -60,5 +79,18 @@ in
         };
       in
         [ (wrapper cfg.package { }) ];
+
+    home.file.".mozilla/${extensionPath}" = mkIf (cfg.extensions != []) (
+      let
+        extensionsEnv = pkgs.buildEnv {
+          name = "hm-firefox-extensions";
+          paths = cfg.extensions;
+        };
+      in
+        {
+          source = "${extensionsEnv}/share/mozilla/${extensionPath}";
+          recursive = true;
+        }
+    );
   };
 }