$ .ajax.done适用于Firefox但不适用于Chrome

I have a very simple AJAX request with jQuery and PHP. This is my code

var request = $.ajax({
    method: "POST",
    url: "Url.php",
    data: { param: valueParam },
    dataType: "html"
});

request.done(function(html) {
   alert('Hello');
});

The code works perfect in Firefox 38.0.5 but not works in Chrome 43.0.2357.124m.

The problem in Chrome is the follow: the "Url.php" returns a pair name-value. If it returns a value distinct than null, then the alert is not displayed (and there isn't any error in the console). But if the url return a null value, the alert is displayed. I tried ctrl+F5 but not works.

In Firefox works good in both cases.

Thanks!

The problem is when I was firing the ajax request. I was doing it in "onchange" (of a input text with jQuery autocomplete) jQuery event. When I select an item of the list of autocomplete, the onchange() event is not raised in Chrome (yes in Firefox).

So I fire the ajax request in jQuery "focusout" event and works in Chrome and Firefox.

Does fail function return something?

request.done(function( msg ) {
  console.log(msg);
});

request.fail(function( jqXHR, textStatus ) {
  console.log("Request failed: "+textStatus);
});