How to edit a JavaScript alert box title?

You can do this in IE:

<script language="VBScript">
Sub myAlert(title, content)
      MsgBox content, 0, title
End Sub
</script>

<script type="text/javascript">
myAlert("My custom title", "Some content");
</script>

(Although, I really wish you couldn't.)

No, it is not possible. You can use a custom javascript alert box.

Found a nice one using jQuery

jQuery Alert Dialogs (Alert, Confirm, & Prompt Replacements)

I Found this Sweetalert for customize header box javascript.

For example

swal({
  title: "Are you sure?",
  text: "You will not be able to recover this imaginary file!",
  type: "warning",
  showCancelButton: true,
  confirmButtonColor: "#DD6B55",
  confirmButtonText: "Yes, delete it!",
  closeOnConfirm: false
},
function(){
  swal("Deleted!", "Your imaginary file has been deleted.", "success");
});

No, you can't.

It's a security/anti-phishing feature.