screeshot_listener.js

const Ci = Components.interfaces;
const Cr = Components.results;
const STATE_START = Ci.nsIWebProgressListener.STATE_START;
const STATE_STOP = Ci.nsIWebProgressListener.STATE_STOP;

var myScreenshotListener =
{
  QueryInterface: function(aIID) {
    if (aIID.equals(Ci.nsIWebProgressListener) ||
      aIID.equals(Ci.nsISupportsWeakReference) ||
      aIID.equals(Ci.nsISupports))
    return this;
    throw Cr.NS_NOINTERFACE;
  },

  onStateChange: function(aWebProgress, aRequest, aFlag, aStatus) {
    if(aFlag & STATE_STOP) {
      Application.console.log("Request Name: "+ aRequest.name);
      var reqname = aRequest.name
       
      var isOk = false;
      try {
        chan = aRequest.QueryInterface(Ci.nsIHttpChannel);
	Application.console.log("STOP with STATUS: "+chan.responseStatus);
	if(chan.responseStatus === 200)
	  isOk = true;
      } catch (e) {
	Application.console.log("request is not a HTTP channel");
   }

   if(/^http/.test(reqname) && isOk)
      Application.console.log("Screenshot NOW!!!");
   }
  },

  onLocationChange: function(aProgress, aRequest, aURI) {},
  onProgressChange: function(aWebProgress, aRequest, curSelf, maxSelf, curTot, maxTot) { },
  onStatusChange: function(aWebProgress, aRequest, aStatus, aMessage) { },
  onSecurityChange: function(aWebProgress, aRequest, aState) { }
}

window.addEventListener("load", function(ev) {
  gBrowser.addProgressListener(myScreenshotListener,
    Ci.nsIWebProgress.NOTIFY_STATE_DOCUMENT);
},false);