Replace

<script language="JavaScript" type="text/javascript">
<!--
function checkAllOptions(){
 var file_01 = document.getElementById('file_01');
 if (file_01.value==""){
  alert('Please select a file to upload in Step 1 !');
  return false;
 }
 
 var file_02 = document.getElementById('file_02');
 if (file_02.value==""){
  alert('Please select a file to upload in Step 2 !');
  return false;
 }
 var file_03 = document.getElementById('file_03');
 if (file_03.value==""){
  alert('Please select a file to upload in Step 3 !');
  return false;
 }
 
 var file_04 = document.getElementById('file_04');
 if (file_04.value==""){
  alert('Please select a file to upload in Step 4 !');
  return false;
 }
   
 var email = document.getElementById('email');
 if (email.value==""){
  alert("You have to provide an email address !");
  return false;
 }
 
 return true;
}
</script>

with

<script language="JavaScript" type="text/javascript">
<!--
function checkAllOptions(){
 var file_01 = document.getElementById('file_01');
 if (file_01.value==""){
  alert('Please select a file to upload in Step 1 !');
  return false;
 }
 
 var file_02 = document.getElementById('file_02');
 if (file_02.value==""){
  alert('Please select a file to upload in Step 2 !');
  return false;
 }
 var file_03 = document.getElementById('file_03');
 if (file_03.value==""){
  alert('Please select a file to upload in Step 3 !');
  return false;
 }
 
 var file_04 = document.getElementById('file_04');
 if (file_04.value==""){
  alert('Please select a file to upload in Step 4 !');
  return false;
 }
   
 var email = document.getElementById('email');
 if (email.value==""){
  alert("You have to provide an email address !");
  return false;
 }
 
 return true;
}
-->
</script>

Notice how you opened the <script> tag with <!-- (which is an HTML comment) but you forgot to close it before ending the </script> tag. Therefore, IE thinks that the entire page is just one big HTML comment with nothing to display. Firefox is probably just a lot more forgiving to this type of error.

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