Comment 14 for bug 1392855

or better,:

<div role="alert" id=motd></div>

<script>
$(document).ready(function() {
        $.get("/motd.json",function(data) {
                var el = document.getElementById("motd");
                var offset = 80 + data.length * 20;
                data.forEach(function(item) {
                        el.innerHTML += "<p>" + item.msg + "</p>";
                        $(".login").css("top",offset.toString() + "px");
                        $("#motd").addClass("alert alert-success");
                });
        })
});
</script>

so it doesn't show if there is no motd.