Javascript works in IE - not in Firefox

I hope this is the right forum...
I have the following script that works good in IE, but when I run it in FF - it doesnt work. I placed some alerts and found what steps it doesnt like, but I cannot see what the issue is.
The basic idea is to take a selected item from dropdownlist8 and add it to listbox1.

alert('aa'); gets displayed in FF but
alert('bb'); does not get executed
so --  document.getElementById('<%=Listbox1.ClientID%>').add(new Option(selectedStatus,selectedValue));

does NOT get executed and FF exits from the function
Any ideas???
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full script:
 
        var statusList = document.getElementById('<%=DropDownList8.ClientID ...