jQuery FullCalendar不会在FireFox 33.0中显示事件问题

Question

I'm trying to set up jQuery FullCalendar by providing data as json feed trough fullcalendar events function. So basically this is my js code:

var calendar = $('#calendar').fullCalendar({

  editable: true,
  firstDay: 1,
  header: {
    left: 'prev, next today',
    center: 'title',
    right: 'month, agendaWeek, agendaDay'
  },

  events: 
  {
    url: 'events.php',
    type: 'POST',
    cache: false,
    error: function() {
      alert('ERROR');
    },
    color: 'blue'
  }
});

And my code from events.php which pulls data from database and makes as JSON feed + converting date to ISO8601 date strings as sayd in documentation.

// Query that retrieves events
$q = "SELECT * FROM evenement ORDER by id";
$r = mysqli_query($dbc, $q);
while ($row = mysqli_fetch_assoc($r)) 
{
    $s = new DateTime($row['start']);
    $e = new DateTime($row['end']);
    $row['start'] = $s->format(DATE_ISO8601);
    $row['end']   = $e->format(DATE_ISO8601);
    $json[] = $row;
}

echo json_encode($json);

In IE11 and Chrome it shows all the events in calendar what are made in DB, but in FireFox the calendar is empty.. And returned response JSON string is:

[
{"id":"7","title":"Rino","start":"2014-10-22T11:55:46+0300","end":"2014-10-22T11:55:46+0300","url":"nav","allDay":"false"},
{"id":"8","title":"Rin","start":"2014-10-22T13:01:45+0300","end":"2014-10-22T13:01:45+0300","url":"Hello","allDay":"false"},
{"id":"9","title":"Rino","start":"2014-10-23T10:06:22+0300","end":"2014-10-23T10:06:22+0300","url":"Hello","allDay":"false"},
{"id":"10","title":"a","start":"2014-10-23T09:42:04+0300","end":"2014-10-23T09:42:04+0300","url":"a","allDay":"false"},
{"id":"11","title":"1","start":"2014-10-23T09:41:55+0300","end":"2014-10-23T09:41:55+0300","url":"1","allDay":"false"},
{"id":"12","title":"sdas","start":"2014-10-23T10:06:14+0300","end":"2014-10-23T10:06:14+0300","url":"1","allDay":"false"},
{"id":"13","title":"Prju","start":"2014-10-16T00:00:00+0300","end":"2014-10-17T00:00:00+0300","url":"undefined","allDay":"false"}
]

What am I missing, why in the Firefox it doesn't show any event? I have also tried clearing cache etc.. but without any results..