how to make java script to work in mozilla firefox remaining all browsers

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C#

ASP.NET

Javascript

hi all...
i have script like this:

<script type="text/javascript">
        function count(txt)
       {
          var textchar='<%= TextBox2.MaxLength %>';
          var textbox=document.getElementById(txt);
          var lbl=document.getElementById('<%= Counlbl.ClientID %>');
          if(textbox.value.length>textchar)
          {
          textbox.value=textbox.value.substrung(0,textchar);
          }
          lbl.innerText=textchar-textbox.value.length+" Characters remaining..";
       }
   </script>

this script working in internet explore but not working in Mozilla Firefox and also i put max length as 120 characters it raises exception after crossing length 1.how can i prevent this exception 2.how can i make this script to work in Firefox can any one please help me
Thanks in Advance