nsIDOMSerializer
to create an instance, use: var domserializer = components.classes["@mozilla.org/xmlextras/xmlserializer;1"] .createinstance(components.interfaces.nsidomserializer); method overview void serializetostream(in nsidomnode root, in nsioutputstream stream, in autf8string charset); astring serializetostring(in nsidomnode root); methods serializetostream() the subtree rooted by the specified element is serialized to a byte stream using the character set specified.
... void serializetostream( in nsidomnode root, in nsioutputstream stream, in autf8string charset ); parameters root the root of the subtree to be serialized.

XMLSerializer - Web APIs
serializetostream() the subtree rooted by the specified element is serialized to a byte stream using the character set specified.