chrome.manifest

content    test-addon    ./
overlay    chrome://browser/content/browser.xul chrome://test-addon/content/overlay.xul

install.rdf

<?xml version="1.0" encoding="UTF-8" ?>
<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:em="http://www.mozilla.org/2004/em-rdf#">
  <Description about="urn:mozilla:install-manifest">
    <em:type>2</em:type>
    <em:id>test@addon</em:id>
    <em:name>test add-on</em:name>
    <em:version>0.1</em:version>
    <em:creator>Nils Maier</em:creator>
    <em:targetApplication>
      <Description>
        <em:id>{ec8030f7-c20a-464f-9b0e-13a3a9e97384}</em:id>
        <em:minVersion>29.0</em:minVersion>
        <em:maxVersion>32.*</em:maxVersion>
      </Description>
    </em:targetApplication>
  </Description>
</RDF>

LICENSE

Any copyright is dedicated to the Public Domain.
http://creativecommons.org/publicdomain/zero/1.0/

overlay.js

// Use strict mode in particular to avoid implicitly var declarations
(function() {
  "use strict";

  // Main runner function for each content window.
  // Similar to SDK page-mod, but without the security boundaries.
  function run(window, document) {
    // jquery setup. per http://stackoverflow.com/a/496970/484441
    $ = function(selector,context) {
      return new jq.fn.init(selector,context || document); 
    };
    $.fn = $.prototype = jq.fn;

    if (document.getElementById("my-example-addon-container"))  {
      return;
    }
    let main = $('<div id="my-example-addon-container">');
    main.appendTo(document.body).text('Example Loaded!');
    main.click(function() { //<--- added this function
      main.text(document.location.href);
    });
    main.css({
      background:'#FFF',color:'#000',position:'absolute',top:0,left:0,padding:8
    });
  };

  const log = Components.utils.reportError.bind(Components.utils);

  // Do not conflict with other add-ons using jquery.
  const jq = jQuery.noConflict(true);

  gBrowser.addEventListener("DOMContentLoaded", function load(evt) {
    try {
      // Call run with this == window ;)
      let doc = evt.target.ownerDocument || evt.target;
      if (!doc.location.href.startsWith("http")) {
        // Do not even attempt to interact with non-http(s)? sites.
        return;
      }
      run.call(doc.defaultView, doc.defaultView, doc);
    }
    catch (ex) {
      log(ex);
    }
  }, true);
})();

overlay.xul

<?xml version="1.0"?>
<overlay id="test-addon-overlay" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
  <script type="text/javascript" src="jquery.js"/>
  <script type="text/javascript" src="overlay.js"/>
</overlay>