Get selected message data in Thunderbird extension

I need to get some e-mail message data in my Thunderbird extension. I found this example on MDN (https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsIMsgMessageService):

var content = "";
  var MessageURI = GetFirstSelectedMessage();
  var MsgService = messenger.messageServiceFromURI(MessageURI);
  var MsgStream =  Components.classes["@mozilla.org/network/sync-stream-listener;1"].createInstance();
  var consumer = MsgStream.QueryInterface(Components.interfaces.nsIInputStream);
  var ScriptInput = Components.classes["@mozilla.org/scriptableinputstream;1"].createInstance();
  var ScriptInputStream = ScriptInput.QueryInterface(Components.interfaces.nsIScriptableInputStream);
  ScriptInputStream.init(consumer);
  try {
    MsgService.streamMessage(MessageURI, MsgStream, msgWindow, null, false, null);
  } catch (ex) {
    alert("error: "+ex)
  }
  ScriptInputStream .available();
  while (ScriptInputStream .available()) {
    content = content + ScriptInputStream .read(512);
  }
  alert(content);

However, when I run it I get the following error:

Timestamp: 2013.06.21. 14:47:21
Error: ReferenceError: GetFirstSelectedMessage is not defined
Source File: chrome://edus_extension/content/messengerOverlay.js
Line: 90

What is this 'GetFirstSelectedMessage' function and how can I get message URI without using it?

This documentation looks fairly outdated. I would suggest:

using gFolderDisplay.selectedMessage (try typing top.opener.gFolderDisplay.selectedMessage in the Error Console),
reading some recent code that uses Services and MailServices so as to simplify your code.

That being said, I don't know what you're trying to achieve but:

you'd certainly be better off using a wrapper such as MsgHdrToMimeMessage (self-reference: http://blog.xulforum.org/index.php?post/2011/01/03/An-overview-of-Thunderbird-Conversations)
if you absolutely, absolutely need to get the raw contents of the message, http://mxr.mozilla.org/comm-central/source/mailnews/db/gloda/modules/mimemsg.js#223 has an example on how to do that (it's the implementation of the said MsgHdrToMimeMessage; by simplifying it, you should be able to get directly the raw data of the message).

Good luck with that, once you get a working sample, please add it to the MDN wiki!

Cheers,

jonathan