Add firefox only on non-darwin systems.

pkgs.firefox's metadata list darwin in badPlatforms.
See: https://github.com/NixOS/nixpkgs/blob/d89f18a17e51532ed5f4d45297b0ddf11e46b9c8/pkgs/applications/networking/browsers/firefox/packages.nix#L21

This applies to firefox-119 and may change

Author: DanGould

flake.nix

@@ -30,9 +30,12 @@
 	  pkgs.clang
           pkgs.corepack_20
           pkgs.nodejs_20
+        ] ++ pkgs.lib.optionals (!pkgs.stdenv.isDarwin) [
+          # Add firefox deps only on non-darwin.
+          # darwin is listed in badPlatforms in pkgs.firefox's meta.
           pkgs.firefox
           pkgs.geckodriver
-	];
+        ];
       in
       {
         defaultPackage = pkgs.rustPlatform.buildRustPackage {