Incompatibility between IE & Firefox when disabling Submit button?

Ok, I've an issue with Internet Explorer, Firefox works though, with disabling the submit button on one click.


Senario:
Firstly, the button is disabled until they click on a select box option, then it enables the submit button, they click the submit button once then it disables itself.


Heres my code:


Function:

function setItems(o)
{ 
 var total = 0;
 for (i = 0; i < o.options.length; i++) {
  if (o.options[i].selected) total++;
  if (total > <? echo $num; ?>) o.options[i].selected = false;
 }
  document.timetable.submit.disabled=false;
}

Code (markup):

Select box:

<select name="setmain[]" multiple="true" size="20" id="setmain[]" onclick="setItems(this)">

Code (markup):

Submit button:

<input type="submit" name="submit" value="Book Set(s)" onclick="this.disabled=true" disabled>

Code (markup):

Problem: People report that when using Internet Explorer, the submit button won't actually submit the form... But when they try in Firefox, it submits!

Any ideas?