The brief points:
If the request header had already been set, then the new value MUST be concatenated to the existing value using a U+002C COMMA followed by a U+0020 SPACE for separation.
UAs MAY give the User-Agent header an initial value, but MUST allow authors to append values to it.
However - After searching through the framework XHR in jQuery they don't allow you to change the User-Agent or Referer headers. The closest thing:
// Set header so the called script knows that it's an XMLHttpRequest
xhr.setRequestHeader("X-Requested-With", "XMLHttpRequest");
I'm leaning towards the opinion that what you want to do is being denied by a security policy in FF - if you want to pass some custom Referer type header you could always do:
xhr.setRequestHeader('X-Alt-Referer', 'http://www.google.com');