Here's a complete working solution.

<form id="form1" runat="server">
    <asp:ScriptManager ID="ScriptManager1" runat="server" />
    <script language="javascript" type="text/javascript">
        function abc() {
            var accHost2 = $get('accSingleData');
            var t;
            for (paneIdx = 0; paneIdx < accHost2.all.length; paneIdx++) {
            if(accHost2.all[paneIdx].id == "accSingleData_Pane_0_content_txtFirstName2_0")
            {
                t = accHost2.all[paneIdx].defaultValue;
                alert(t);
                }
            }
        }


</script>
 <div>
 <ajaxToolkit:Accordion ID="accSingleData" runat="server" Enabled="True" SelectedIndex="0">
    <HeaderTemplate>
    </HeaderTemplate>
    <ContentTemplate>
        <div>
        <asp:TextBox name="axd" ID="txtFirstName2" runat="server" Text="a1" />
        </div>
    </ContentTemplate>
</ajaxToolkit:Accordion>
 

<br />
<asp:Button ID="Button1" runat="server" Text="Button" OnClientClick="abc();"
            onclick="Button1_Click" />
    </div>
    </form>


----------------------------------------------
code-behind
-----------------------------------------------
protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            accSingleData.DataSource = new string[] { "one", "two", "three" };
            accSingleData.DataBind();
        }
    }
    protected void Button1_Click(object sender, EventArgs e)
    {


    }
    protected void accSingleData_ItemCommand(object sender, CommandEventArgs e)
    {
        

    }
    protected void accSingleData_ItemDataBound(object sender, AjaxControlToolkit.AccordionItemEventArgs e)
    {
       

       



    }