How to do a cross domain request in Firefox OS

crossdomainrequest.js

/*
  YOU'LL BE ABLE TO PERFORM A CROSS DOMAIN REQUEST IF YOUR APP IS CERTIFIED OR PRIVILEDGED.
  FOR MORE INFO CHECK THE PERMISSIONS MATRIX: https://docs.google.com/spreadsheet/ccc?key=0Akyz_Bqjgf5pdENVekxYRjBTX0dCXzItMnRyUU1RQ0E#gid=0
  
  First you'll need to setup the permission for doing the cross domain request on your application manifest.
  
  Go to your app manifest, to the permissions section and add the following:
  
  "systemXHR": {}
  
  Now let's create the xhr object, it needs a special parameter, take a look:
*/
 
var xhr = new XMLHttpRequest({ mozSystem: true });
 
// Use the xhr object as usual ...
 
// When checking for the result, you'll look for response status 200 OR (AND THIS IS IMPORTANT) 0
xhr.onload = function(e) {
  if (xhr.status === 200 || xhr.status === 0) {
    // Do your stuff here
  } else {
    // An error happened 
  }
}

Did you know any way to do web services request what doesnt need modify server config ? i have an existent web service and i want to connect the firefox app to that.

Hosted apps should work without systemXHR and mozSystem, shouldn't they?