Firefox Doesn't Like AppendChild()

I have 2 list boxes - one to fill the second one based on the selection, or
move all items. You can remove the selection (or all items) from the second
one to place it back in the first one. I expanded my horizons and thought
to use script based on the js node operation appendChild(). It seemed so
clean and easy to follow. Works beautifully in IE, but Firefox sees the
value for a nanosecond but doesn't put it in the box. This is the relevant
js:

function addSide(){
var addIndex = document.forms[0].sides.selectedIndex;

if (addIndex >= 0)
{
document.forms[0].selectedSides.appendChild(document.forms[0].sides.options(addIndex));
}}

function delSide(){
var selIndex = document.forms[0].selectedSides.selectedIndex;
if (selIndex >= 0)
{
document.forms[0].sides.appendChild(document.forms[0].selectedSides.options(selIndex))
}}

function addAll(){
var len = document.forms[0].sides.length -1;
for(i=len; i>=0; i--){
document.forms[0].selectedSides.appendChild(document.forms[0].sides(i));
}}

function delAll(){
var len = document.forms[0].selectedSides.length -1;
for(i=len; i>=0; i--){
document.forms[0].sides.appendChild(document.forms[0].selectedSides(i));
}}

Is there a way to make it work for FF, or do I need to just start over and
not use appendChild()? Is there a better way?