function getMediaPlayer() { alert("called"); var mainWindow = window.QueryInterface(Components.interfaces.nsIInterfaceRequestor) .getInterface(Components.interfaces.nsIWebNavigation) .QueryInterface(Components.interfaces.nsIDocShellTreeItem) .rootTreeItem .QueryInterface(Components.interfaces.nsIInterfaceRequestor) .getInterface(Components.interfaces.nsIDOMWindow); var num = mainWindow.gBrowser.mPanelContainer.childNodes.length; for (var i = 0; i <=num; i++) { alert("index:"+i); var b = mainWindow.gBrowser.getBrowserAtIndex(i); if (b.content.document.getElementById("mediaplayer")) { alert("found"); } }}