jQuery $.ajax(), $.post sending "OPTIONS" as REQUEST_METHOD in Firefox

The reason for the error is the same origin policy. It only allows you to do XMLHTTPRequests to your own domain. See if you can use a JSONP callback instead:

$.getJSON( 'http://<url>/api.php?callback=?', function ( data ) { alert ( data ); } );

I used the following code on Django side to interpret the OPTIONS request and to set the required Access-Control headers. After this my cross domain requests from Firefox started working. As said before, the browser first sends the OPTIONS request and then immediately after that the POST/GET

def send_data(request):
    if request.method == "OPTIONS": 
        response = HttpResponse()
        response['Access-Control-Allow-Origin'] = '*'
        response['Access-Control-Allow-Methods'] = 'POST, GET, OPTIONS'
        response['Access-Control-Max-Age'] = 1000
        # note that '*' is not valid for Access-Control-Allow-Headers
        response['Access-Control-Allow-Headers'] = 'origin, x-csrftoken, content-type, accept'
        return response
    if request.method == "POST":
        # ... 

Edit: it seems to be that at least in some cases you also need to add the same Access-Control headers to the actual response. This can be a little bit confusing, since the request seems to succeed, but Firefox does not pass the contents of the response to the Javascript.