Possible bugs in Mozilla Jetpack?

I've noticed a couple of things that don't work in Mozilla Jetpack. One is:

var serializer = new XMLSerializer( );

This line works fine in the console -- it works in Firebug. But for some reason, in Jetpack, I get "XMLSerializer is undefined."

Fortunately, I have a workaround. The workaround is:

var serializerClass = "@mozilla.org/xmlextras/xmlserializer;1";
var serializer =
    Components.classes[serializerClass];
var serializerInstance =
    serializer.createInstance(Components.interfaces.nsIDOMSerializer);

The second thing that doesn't work for me in Jetpack is writing to a document object using document.write():

  jetpack.tabs.focused.contentWindow.open(); // works
  var doc = jetpack.tabs.focused.contentDocument;
  // This part doesn't work:
  doc.open( );
  doc.write( formattedContent );
  doc.close( );

It also doesn't work if I try to do

  win = jetpack.tabs.focused.contentWindow.open();
 doc = win.document;
 doc.open( );
 // etc.

Jetpack will open() a new window in a fresh tab but won't give me a reference to the new window's document object. The window stays blank -- I can't write to it.

If anyone has a workaround to this, please let me know. It seems odd that I can't create a new page from Jetpack.