disable controls in mozilla firefox using javascript

I used this code but i was unable to disabled the controls in mozilla firefox.

<script type="text/javascript">
function DisableControls(el)
{
    try
    {
        el.disabled = true;
    }
    catch(E){}
    if (el.childNodes && el.childNodes.length > 0) {
        for (var x = 0; x < el.childNodes.length; x++) {
            DisableControls(el.childNodes[x]);
            }
    }
}
DisableControls(document.getElementById("<%=pnlQuestions.ClientID %>"));
</script>

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