Trying to set focus programmatically does not work with Firefox

I have a problem in Firefox trying to set the focus to a control using jqueryjavascript. While my specific problem is quite involved, even the simple test case below does not work for me. The input is found, its value is changed but the input is not focused.
Issue present with Firefox only

Tried with Firefox 44.0.2 on Windows 7
It works as it should with MS IE 10 and Chrome

$().ready(function() {
  $("#I").val(3);
  $("#I").focus(); // JQuery focus - does not work
  
  var ctl = document.getElementById('I')
  ctl.value='DOM';
  ctl.focus(); // DOM Native focus - does not work
})

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<input type=text id=I>