Obsolete
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
The ExtensionManager follows the nsIExtensionManager API.
Note: This API is obsolete starting in Gecko 2.0, and has been replaced by the new Add-on Manager.
Examples
Here is how to retrive all the extensions installed:
var em = Cc['@mozilla.org/extensions/manager;1']
.getService(Ci.nsIExtensionManager);
const nsIUpdateItem = Ci.nsIUpdateItem;
var extension_type = nsIUpdateItem.TYPE_EXTENSION;
items = em.getItemList(extension_type, {});
items.forEach(function(item, index, array) {
alert(item.name + " / " + item.id + " version: " + item.version);
});