javascript - Access Control Request Headers, is added to header in AJAX request with jQuery

I would like to add a custom header to an AJAX POST request from jQuery.

I have tried this:

$.ajax({
    type: 'POST',

    url: url,
    headers: {
        "My-First-Header":"first value",
        "My-Second-Header":"second value"
    }
    //OR
    //beforeSend: function(xhr) { 
    //  xhr.setRequestHeader("My-First-Header", "first value"); 
    //  xhr.setRequestHeader("My-Second-Header", "second value"); 
    //}

}).done(function(data) { 
    alert(data);
});

When I send this request and I watch with FireBug, I see this header:

OPTIONS xxxx/yyyy HTTP/1.1
Host: 127.0.0.1:6666
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:11.0) Gecko/20100101 Firefox/11.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,/;q=0.8
Accept-Language: fr,fr-fr;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip, deflate
Connection: keep-alive
Origin: null
Access-Control-Request-Method: POST
Access-Control-Request-Headers: my-first-header,my-second-header
Pragma: no-cache
Cache-Control: no-cache

Why do my custom headers go to Access-Control-Request-Headers:

Access-Control-Request-Headers: my-first-header,my-second-header

I was expecting a header values like this:

My-First-Header: first value
My-Second-Header: second value

Is it possible?

Answer

What you saw in Firefox was not the actual request; note that the HTTP method is OPTIONS, not POST. It was actually the 'pre-flight' request that the browser makes to determine whether a cross-domain AJAX request should be allowed:

http://www.w3.org/TR/cors/

The Access-Control-Request-Headers header in the pre-flight request includes the list of headers in the actual request. The server is then expected to report back whether these headers are supported in this context or not, before the browser submits the actual request.