XMLDocument.save("localfile.xml") <possible solutions>

XMLDocument.prototype.save = function(localfile) {
   var C = Components.classes, I = Components.interfaces;

   var localFile = 
C["@mozilla.org/file/local;1"].createInstance(I.nsILocalFile);
   localFile.initWithPath(localfile);
   if (!localFile.exists()) localFile.create(localFile.NORMAL_FILE_TYPE, 
0644);

   var outputStream = 
C["@mozilla.org/network/file-output-stream;1"].createInstance(I.nsIFileOutputStream);
   outputStream.init(localFile, 0x04 | 0x08 | 0x20, 0644, 0);
   // 
<http://xulplanet.com/references/objref/XMLSerializer.html#method_serializeToStream>
   (new XMLSerializer).serializeToStream(this, outputStream, "UTF-8");
   outputStream.close();
}

XMLDocument.prototoype.save = function(localfile) {
   var C = Components.classes, I = Components.interfaces;

   var fos = C["@mozilla.org/file/local;1"].createInstance(I.nsILocalFile);
   fos.initWithPath(localfile);
   if (!fos.exists()) fos.create(fos.NORMAL_FILE_TYPE, 0644);

   var charset = "UTF-8"; // can be any character encoding name that 
Mozilla supports
   var os = 
C["@mozilla.org/intl/converter-output-stream;1"].createInstance(I.nsIConverterOutputStream);
   os.init(fos, charset, 0, 0x0000);
   //os.writeString("Umlaute: \u00FC \u00E4\n"); // direct JSString 
access, not needed here.
   //os.writeString("Hebrew:  \u05D0 \u05D1\n");

   (new XMLSerializer).serializeToStream(this, os, charset);
   os.close();
}