So I just learned how to do this… what a time saver this will be for AJAX development. Be warned that this allows cross-domain XHR’s to and from any site! This is a major security risk.
1. Close Firefox
2. Edit the file prefs.js in your Firefox user profile folder
3. Add the following line anywhere in the file
user_pref(“capability.policy.default.XMLHttpRequest.open”, “allAccess”);
4. Save the file and re-open Firefox. You can now risk your life and limb by doing XHR’s to whatever domains you want – congratulations!