xmlhttp无刷新二级联动支持firefox

javascript代码:

<script language="javascript" type="text/javascript"> 
//jb函数会根据不同的浏览器初始化个xmlhttp对象 
function jb() 
script> 

aspx代码：

        <tr>
          <td height="30" align="center">
              <asp:DropDownList ID="ddlyears" runat="server" AppendDataBoundItems="True">
                  <asp:ListItem Value="">请选择年份</asp:ListItem>
              </asp:DropDownList>                  
              </td>
          <td align="center"><asp:DropDownList ID="ddlnos" runat="server" AppendDataBoundItems="True">
                <asp:ListItem Value="">请选择刊号</asp:ListItem>
              </asp:DropDownList></td>
        </tr>

c#代码：

protected void Page_Load(object sender, EventArgs e)
    }