How can a Firefox extension get its own version number programmatically?

Question

How do I programatically get my own Firefox extension's version number with Javascript?

My extension has an install.rdf file containing the version number similar to below. I want to extract the contents of the <em:version> tag.

<?xml version="1.0" encoding="UTF-8"?>
<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#" 
 xmlns:em="http://www.mozilla.org/2004/em-rdf#">
  <Description about="urn:mozilla:install-manifest">
    ...
    <em:version>1.0</em:version>
    ...
  </Description>
</RDF>

Answer 1

In Firefox 4 (Gecko 2) the API has changed, so if you need to port to Firefox 4, this is the code (from here):

try {
    // Firefox 4 and later; Mozilla 2 and later
    Components.utils.import("resource://gre/modules/AddonManager.jsm");
    AddonManager.getAddonByID("[email protected]", function(addon) {
        alert("My extension's version is " + addon.version);
  });
}
catch (ex) {
    // Firefox 3.6 and before; Mozilla 1.9.2 and before
    var em = Components.classes["@mozilla.org/extensions/manager;1"]
             .getService(Components.interfaces.nsIExtensionManager);
    var addon = em.getItemForID("[email protected]");
    alert("My extension's version is " + addon.version);
}

Answer 2

I've not got the full answer, but I found the Extended extension and had a look at the source code as it seemed like a good starting point, and from Googling some of the methods in that I found this snippet on MDC. The key bit of code would seem to be this:

var gExtensionManager = Components.classes["@mozilla.org/extensions/manager;1"]
                        .getService(Components.interfaces.nsIExtensionManager);
var current = gExtensionManager.getItemForID("[email protected]").version;

You would have to replace [email protected] with the appropriate ID for your extension.

Firefox 4 requires different code, see the other answer.