Question

The responseText is empty in Firefox, but ok in Internet Explorer.
I also log the response before returning to the client so I can see my response there.

This is my request , I added a setTimeout but this also not helping.

var ajaxUrl = "./ajaxHandlers/ajax-handler.php";

var myAjax = new Ajax.Request(
  ajaxUrl,
  {
    method: 'post', 
    parameters: params, 
    onComplete: function(response)
    { 
      setTimeout(handleResponse(response,callback) ,5000);
    } 
 });



 function handleResponse(response,callback)
 {
  alert(response.responseText);

  try
  {
     eval("var r = " + response.responseText);
  }
  catch (e)
  {
     alert("EXCEPTION = " + e.constructor);

     showError("error evaluating response : Response text:<br/>" + response.responseText);
     var r = new Object();
     r.message = 'Error evaluating response';
     r.status = 'error';
     if (typeof callback == 'function') callback(r);
     return;
  }
}

Answer 1

In the onComplete your callback variable is not defined so this causes your problem :

onComplete: function(response)
{
      handleResponse(response);
} 

or

onComplete: function(response, callback)
{
      handleResponse(response, callback);
} 

Edit:
Here is the full code I have. It is working absolutely ok on FF 3.5.10 .. hmm not the latest but should be the same. I can confirm the ajax is synchronous because of the order of the console.info output statements :

var ajaxUrl = "./ajaxHandlers/ajax-handler.php";
console.info('1');

var myAjax = new Ajax.Request(
  ajaxUrl,
  {
    method: 'post',
    asynchronous: false, 
    parameters: {}, 
    onComplete: function(response, callback)
    {
      handleResponse(response,callback);
    } 
   }
 );

console.info('2');

 function handleResponse(response, callback)
 {
 console.info(response.responseText);

  try
  {
     eval("var r = " + response.responseText);
  }
  catch (e)
  {
     alert("EXCEPTION = " + e.constructor);

     showError("error evaluating response : Response text:<br/>" + response.responseText);
     var r = new Object();
     r.message = 'Error evaluating response';
     r.status = 'error';
     if (typeof callback == 'function') callback(r);
     return;
  }
}