Why does Firebug delay console output?

Question

I have a simple promise based $.ajax post loop. The code works as expected, so I am just curious. (All data is shown in the console after the done() function was executed). However Firebug displays data from the PHP script after the output of the done() method. Why is this? Does Firebug do something while the code continues to run?

jQuery

var promises = [];
for (i = 0; i < tochange.length; i++) {
  var promise = $.ajax({
    type:"POST",
    url:"updateLevel.php",
    data: { graph_plot: tochange[i], new_plot:changeto[i]}
  }).done(function(data){
    console.log(data);
  });

  promises.push(promise);
}

$.when.apply(null, promises).done(function() {
  console.log('All done')
});

PHP

require_once(dirname(__FILE__) . '/../services/FirePHP.class.php');
require_once(dirname(__FILE__) . '/../services/fb.php');

ob_start();

$firephp = FirePHP::getInstance(true);
$graph_plot = $_POST['graph_plot'];
$new_plot = $_POST['new_plot'];

if ($graph_plot && $new_plot) {
  fb("$graph_plot, $new_plot");
}