I find ResourcesAction as written clumsy to configure and I am sure it
could be improved a bit: right now it is hardcoded to link for URL
links in the Resources/ system folder, which means there can only be
one such button at a time. Better style: have some class implementing
Presenter.Toolbar (not a SystemAction which is a singleton!) named
e.g. DropDownButton; then in the layer e.g.:

<folder name="Toolbars">
  <folder name="Data">
    <file name="ffj-resources.instance">
      <attr name="instanceCreate"
            methodvalue="org.netbeans.modules.url.DropDownButton.create"/>
      <attr name="folder" stringvalue="DropDownButton-data/ffj-resources"/>
    </file>
  </folder>
</folder>
<folder name="DropDownButton-data">
  <folder name="ffj-resources">
    <file name="something.url">http://..../</file>
    <!-- etc. -->
  </folder>
</folder>

where

public class DropDownButton implements Presenter.Toolbar {
    public static DropDownButton create(FileObject fo) {
        String folder = (String)fo.getAttribute("folder");
        // find in SystemFileSystem, configure from there...
        Node n = DataObject.find(fo).getNodeDelegate();
        // use n.getDisplayName() & n.getIcon() for presenters
    }
}

Not really much more work to code, and much more flexible.