Currently I don't get any code completion for the following code:

    var file = Components.classes["@mozilla.org/file/local;1"]
                    .createInstance(Components.interfaces.nsILocalFile);
    file.initWithPath(filePath); // I would like to be able to enum members of the nsILocalFile interface

The API reference can be found at http://www.xulplanet.com/references/xpcomref/. There is an IDE (http://www.spket.com) that supports Mozilla better.