Sample code for progress listener.

progress_listeners.js

/*
 * progress_listener.js
 * Kosei Moriyama <[email protected]>
 *
 * Sample code for Firefox Extension.
 * Do something when uri is changed using progress listener.
 */

window.addEventListener('load', function() { myExtension.init(); }, false);
window.addEventListener("unload", function() { myExtension.uninit(); }, false);

var myExtension = {
  // store old url
  oldUrl: null,

  // implement nsIWebProgress interface
  urlBarListener: {
    QueryInterface: function(aIID) {
      if (aIID.equals(Components.interfaces.nsIWebProgressListener) ||
          aIID.equals(Components.interfaces.nsISupportsWeakReference) ||
          aIID.equals(Components.interfaces.nsISupports))
        return this;
      throw Components.results.NS_NOINTERFACE;
    },

    // called when uri of document of current tab is changed
    onLocationChange: function(aProgress, aRequest, aUri) {
      myExtension.processNewUrl(aUri);
    },

    onStateChange: function() {},
    onProgressChange: function() {},
    onStatusChange: function() {},
    onSecurityChange: function() {},
    onLinkIconAvailable: function() {}
  },

  // add progress listener at first
  init: function() {
    gBrowser.addProgressListener(myExtension.urlBarListener, Components.interfaces.nsIWebProgress.NOTIFY_STATE_DOCUMENT);
  },

  // remove progress listener at last
  uninit: function() {
    gBrowser.removeProgressListener(myExtension.urlBarListener);
  },

  // do something when uri is changed
  processNewUrl: function(aUri) {
    if (aUri.spec == myExtension.oldUrl)
      return;
   
    // write your code here

    myExtension.oldUrl = aUri.spec;
  }
};