联级下拉选择框IE5不能实现，求救。

function bindBigCategory(BigCategory)
{
var sl_BigCategory = document.getElementById(BigCategory)
var BigCategory = new Array("安全、防护");

var _opt = document.createElement("OPTION");
_opt.value = "-1";
_opt.text = "请选择大类";
sl_BigCategory.add(_opt);

for(var i = 0; i < BigCategory.length; i ++)
{
var opt = document.createElement("OPTION");
opt.value = i;
opt.text = BigCategory[i];
sl_BigCategory.add(opt);
}
return;
}

function bindSmallCategory(BigCategory, SmallCategory)
{
var BigCategoryCode = document.getElementById(BigCategory).value;

var sl_SmallCategory = document.getElementById(SmallCategory);
while(sl_SmallCategory.length > 0)
{
sl_SmallCategory.options.remove(0);
}

var SmallCategory = new Array(
new Array("锁具", "保险柜"),
new Array("纸浆", "文化用纸")
)
var _opt = document.createElement("OPTION");
_opt.value = "-1";
_opt.text = "请选择小类";
sl_SmallCategory.add(_opt);

if(BigCategoryCode != "-1")
{
for(var i = 0; i < SmallCategory[BigCategoryCode].length; i ++)
{
var opt = document.createElement("OPTION");
opt.value = i;
opt.text = SmallCategory[BigCategoryCode][i];
sl_SmallCategory.add(opt);
}
}

return;
}

<SELECT id="sl_BigCategory" onchange="bindSmallCategory('sl_BigCategory', 'sl_SmallCategory');" name="sl_BigCategory">
<SCRIPT type="text/javascript">
bindBigCategory("sl_BigCategory");
</SCRIPT>
</SELECT>


<SELECT id="sl_SmallCategory" name="sl_SmallCategory">
<SCRIPT type="text/javascript">
bindSmallCategory("sl_BigCategory", "sl_SmallCategory");
</SCRIPT>
</SELECT>

以上脚本在IE6中可以运行，在IE5中就无法运行，我把sl_BigCategory.add(_opt);中的add改成appendChild，在firefox中可以运行，但是在IE6中不可以运行．但是sl_SmallCategory.options.remove(0);在firefox中报错说remove不是函数．

我想在IE5,IE6以及firefox中都能运行．请问该怎么写？