在用户定义的变量上获得selecteindex -适用于IE而不是Firefox

我有以下函数，根据用户的选择填充HTML文本字段。由于引用的数量可以是1到多，所以我传入了它们正在更新的引用的id。

我得到了一行的错误:

var IndexOfReferenceSelect = ReferenceSelect.selectedIndex;

表明ReferenceSelect是null。如何在Firefox浏览器中设置ReferenceSelect ?我认为使用getElementById会起作用。在IE中可以，但在Firefox中不行。

function SelectReference(ReferenceNum){
    var i;
    var ReferenceSelect = document.getElementById("Reference_" + ReferenceNum);
    var Author      = document.getElementById("Author_Last_Name_" + ReferenceNum);
    var Title       = document.getElementById("Title_" + ReferenceNum);
    var Edition     = document.getElementById("Edition_" + ReferenceNum);
    var Publisher   = document.getElementById("Publisher_" + ReferenceNum);
    var Year1       = document.getElementById("Year_" + ReferenceNum);
    var Pages       = document.getElementById("Pages_" + ReferenceNum);
    var ReferenceList = new Array(<%=strBigListOfOptionTagGroups%>);
    var IndexOfReferenceSelect = ReferenceSelect.selectedIndex;
    var IndexOfReferences = ReferenceList[IndexOfReferenceSelect].split("|");
    Author.value    = IndexOfReferences[0];
    Title.value     = IndexOfReferences[1];
    Edition.value   = IndexOfReferences[2];
    Publisher.value = IndexOfReferences[3];
    Year1.value     = IndexOfReferences[4];
    if (IndexOfReferenceSelect == 0){
        Pages.value = '';
    }
}