When did it work? nsIDictionary hasn't been part of the Firefox code since Firefox 3.5 or so. Also, we disabled .enablePrivilege for web content, which may affect your testcase, although I'd expect a different exception in that case.
Can someone please update the article to say that it is no longer part of Firefox? (see: https://developer.mozilla.org/en/XPCOM_Interface_Reference/nsIDictionary) As for the enablePrivilege, I need to call this for *all* of the XPCOM interfaces otherwise Firefox says that I don't have permissions to access any of the methods/attributes for the objects.
I updated the article.