[Aetae]

В ff можно вот так:

<div class="N" id="3"> </div>
<div class="N" id="1"> </div>
<div class="N" id="12"> </div>
<div class="N" id="4"> </div>
<div class="N" id="2"> </div>

<script>
alert(
  Array.reduce( 
    document.querySelectorAll(".N"), 
    (previous, {id:current}) => Math.max(current, previous), 
    -Infinity
  )
);
</script>