Nix / NixOS

Hi,

I want to sort my bookmarks in Firefox with home-manager into folders, but fail.

Simple example:

firefox = {
      profiles."user" = {
        bookmarks = [
          {
            name = "Nix";
            toolbar = true;
            bookmarks = [
              {
                name = "NixOS Search";
                url = "https://search.nixos.org/packages";
              }
              {
                name = "NixOS Options";
                url = "https://nixos.org/manual/nixos/unstable/options";
              }
              {
                name = "Home-Manager Options";
                url = "https://nix-community.github.io/home-manager/options.xhtml";
              }
              {
                name = "Home-Manager Options Search";
                url = "https://home-manager-options.extranix.com/";
              }
            ];
          }
        ];
      };

My assumption was that I get a folder "Nix" in the bookmarks toolbar that contains the four bookmarks. But instead the four bookmarks are added to the toolbar side-by-side without being in a folder.

How can I achieve that?