How do I enable UniversalXPConnect for a particular web page?

Question

I need to test various web services which are posts that take an uploaded file as the content of the body. To do this I'd like to do quick tests using ajax call. I found the following page which describes how to do this: http://www.captain.at/ajax-file-upload.php However, it requires that the page have "UniversalXPConnect" privileges in firefox.

How do I enable that privilege? I tried editing prefs.js and adding:

user_pref("capability.principal.foo.id", "http://localhost:8080/access/index.html");
user_pref("capability.principal.foo.granted", "UniversalXPConnect");

which should give access to the page http://localhost:8080/access/index.html. But, it doesn't seem to work.