First, this .xpi is a very good one to study, as it's something that I'll copy many things from:

    http://www.raphinou.com/cgi-bin/myowncss/main.cgi


    - the contents.rdf and myowncss-overlay.xul are delightfully short

    - it looks like the first real thing it does is run: document.getElementById("content");
        Using the DOM explorer, it looks like this is the tabbrowser#content tag

                http://lxr.mozilla.org/seamonkey/source/toolkit/content/widgets/tabbrowser.xml
        



Similarly, this one should be studied:

    http://extensionroom.mozdev.org/more-info/uriid