.done is not a function

I've another problem. I get an error in FireFox and I don't know what my fault is. I always did it like this way and I never got an error. I already check lower/uppercase mistakes but I can't find anything.

Thanks

$.ajax({type: "POST", url: "ajax/check_username.php", data: {username: username}}).done is not a function

<script type="text/javascript">
$(document).ready(function(){
    $("#username").keyup(function(){
        var username = $("#username").val();
        $(".usernameFeedback").fadeIn("fast");

        $.ajax({
            type: "POST",
            url: "ajax/check_username.php",
            data: { username: username }
        }).done(function( msg ) {
            $("#loadingImage").hide();
            if(msg.status != "error")
                {
                    if(msg.available == "yes")
                    {
                        $(".usernameFeedback span").text(msg.message);
                        $(".usernameFeedback span").removeClass("notok");
                        $(".usernameFeedback span").addClass("ok");
                    }
                    else
                    {
                        $(".usernameFeedback span").text(msg.message);
                        $(".usernameFeedback span").addClass("notok");
                    }
                }
        });
        return(false);
    })
});
</script>