Java Script not working in Firefox

Question

i am using below javascript in my applicaion. its working fine in IE but not working Properly in firefox.

<script type="text/javascript">
                                                          function CountChars(CharLimit,InputID,OutputID) {
                                                              var enterchar;
                                                              var textboxvalue;
                                                              var remainingchars;
                                                              enterchar = document.getElementById(InputID).value.length;
                                                              textboxvalue = document.getElementById(InputID).value;
                                                              remainingchars = CharLimit - enterchar;
                                                              document.getElementById(OutputID).innerHTML = remainingchars + " Characters Left(max:156 Characters)";
                                                               if (remainingchars <= 0) {
															   document.getElementById(OutputID).innerHTML="0 Characters Left(max:156 Characters)";
                                                                   alert("you have reached the limit");
                                                                   document.getElementById(InputID).innerHTML = textboxvalue.substring(0, CharLimit);
                                                               }
                }
                                                              
</script>

its working partially. its calculating the remaining char but substring fun is not working in firefox.

Aman Gandhi