Probaj ovako provereno radi i u IE8 i u Mozilla Firefox-u:
<asp:UpdatePanel ID="pnlUpdate" runat="server">
<ContentTemplate>
<asp:GridView ID="Grid1">
<Columns>
<asp:TemplateField HeaderText="Neki heder za grid 2">
<ItemTemplate>
<asp:Panel ID="pnlPrvi" runat="server" CssClass="panelCollapse">
<asp:Image ID="imgCollapsible" ImageUrl="~/Images/botleft.png" Style="margin-right: 5px;" runat="server" />
<asp:Label ID="lblName1" runat="server" CssClass="LabelStyle" Text='<%# Bind("NAME1") %>'></asp:Label>
<asp:Label ID="lblName2" runat="server" CssClass="LabelStyle" Text='<%# Bind("NAME2") %>'></asp:Label>
.
.
.
.
</asp:Panel>
<asp:Panel ID="pnlDrugi" runat="server">
<asp:GridView ID="Grid2">
<Columns>
.
.
.
.
</Columns>
</asp:GridView>
</asp:Panel>
<ajaxToolkit:CollapsiblePanelExtender ID="cpe" runat="Server" TargetControlID="pnlDrugi"
CollapsedSize="0" Collapsed="False" ExpandControlID="pnlPrvi" CollapseControlID="pnlPrvi"
AutoCollapse="False" AutoExpand="False" ScrollContents="false" ImageControlID="imgCollapsible"
ExpandedImage="~/Images/minus.png" CollapsedImage="~/Images/plus.png"
ExpandDirection="Vertical" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</ContentTemplate>
</asp:UpdatePanel>
Tu ti je prikazano kako treba da uradiš sa dva grida ti dalje proširi na tri.
ali u mozilli 
, dok u IE8 koji mi je pored mozile standard nece.