onPageLoad: function(aEvent) {
        var doc = aEvent.originalTarget; // doc is document that triggered the event
        var win = document.defaultView; // win is the window for the doc

        var jsLoader = Components.classes["@mozilla.org/moz/jssubscript-loader;1"].getService(Components.interfaces.mozIJSSubScriptLoader);
        jsLoader.loadSubScript("chrome://myExt/content/jquery-1.8.0.js");

if($("#id1").length>0) // length is always 0
// do something

var x  = document.getElementById("id1"); //plain old javascript and it works