javascript problem! with browser

[savagestudio]

hello i have a probleme with a javascript that work on IE, safari,Chrome but doesn't in firefox or opera :

it is a simple script that erase and rewrite the content of 2 select list/menu from an 2 array.. I will apreciate some help on this one, here is the code :
i have put all the script without the body part and have highlighted where is the problem:

the think is that it work perfectly in other browser than firefox and opera and sea monkey...!

<script language="javascript">
//(arrays are empty for space)
var sub_cat_array = new Array(); //1st array for 2nd list
var sub_cat_array2 = new Array(); //2nd array for 3rd list

window.onload=function(){
document.getElementById("hidden_elements").style.display="none";
document.getElementById("hidden_elements2").style.display="none";
document.getElementById("markedslist").onchange=selcted;
document.getElementById("subcat1").onchange=selcted2;
    }
    
function selcted(){           document.getElementById("hidden_elements").style.display="none";
document.getElementById("hidden_elements2").style.display="none";
if(document.getElementById("markedslist").value !="all"){
document.getElementById("hidden_elements").style.display="block";
}else{    
        document.getElementById("hidden_elements").style.display="none";
        document.getElementById("hidden_elements2").style.display="none";
        }
        var markedslist=this.value;
        var sub_cat=document.getElementById("subcat1");    
        sub_cat.options.length=0;

//PROBLEM START HERE
        for(var i=0;i< sub_cat_array.length; i++){
 var st = sub_cat_array[i][0];
        if(st==markedslist){
        var opt = new Option(sub_cat_array[i][1]);;
                opt.value=sub_cat_array[i][2];
                try{
                    sub_cat.add(opt,opt.value);
                    }catch(e){
                        sub_cat.add(opt);
                        }
                }
            
            }
        } 
// FINISH HERE it is the same for the function selcted2 below


    function selcted2(){
        if(document.getElementById("subcat1").value != "all"){
        document.getElementById("hidden_elements2").style.display="block";
        }else{    document.getElementById("hidden_elements2").style.display="none";
        }    
            var subcat1=this.value;
        var sub_cat2=document.getElementById("subcat2");    
        sub_cat2.options.length=0;
        
        for(var i=0;i< sub_cat_array2.length; i++){
            var st2 = sub_cat_array2[i][0];
            if(st2==subcat1){
                var opt2 = new Option(sub_cat_array2[i][1]);
                opt2.value=sub_cat_array2[i][2];
                try{
                    sub_cat2.add(opt2,opt2.value);
                    }catch(e){
                        sub_cat2.add(opt2);
                        }
                }
            
            }
        }
</script>

[savagestudio]

here the body part if needed
It is a php file that is include in the main page so there is no headers..

<form id="form1" name="form1" method="post" action="">

<table width="778" border="0" id="markedsearch" cellpadding="0" cellspacing="5">
<tr>
    <td width="70" rowspan="2" align="center" valign="top" bgcolor="#000000" ><img src="<?php echo HTTP_SERVER;?>images/sok.jpg" width="70" height="50" /></td>
    <td width="200" align="left" ><select name="groupe1" class="listSize" id="markedslist">
      <option value="all">Alle rubrikker</option>
      <option  value="ein">Eiendom</option>
      <option value="bil">Bill</option>
      <option  value="boa">B&aring;t</option>
      <option value="job">Jobb</option>
      <option  value="tor">Torget</option>
    </select></td>
    <td width="172" align="left" >  <div id="hidden_elements"><select class="listSize" name="subcat1" size="1" id="subcat1">
    </select>  </div>  </td>
    <td width="172" align="left" ><div id="hidden_elements2"><select class="listSize" name="subcat2" size="1" id="subcat2">
    </select></div></td>
    <td width="142" rowspan="2" align="center" valign="middle" ><input type="submit" name="submitmarked" id="submitmarked" class="submitmarked" value="Søk" /></td>
    </tr>
 <tr align="left">
      <td colspan="3" align="center" valign="middle"><input name="criteria" class="searchimput_marked" type="text" id="criteria" size="60" />          </td>
    </tr>

</table>
</form>

[savagestudio]

the answer is:

 sub_cat.options[sub_cat.length] = new Option(sub_cat_array[i][1],sub_cat_array[i][2]);

instead of:

sub_cat2.add(opt2,opt2.value);