Panel default button property is not working in Firefox

Hi
I have a panel inside a usercontrol i adjusted default button property to btnsave for this panel
this property is working in IE but it doesnot working in Firefox, in  Firefox another button is executin when i press enter

<asp:Panel runat="server" ID="PanelCreateEdit" DefaultButton="btnSave"> <div class="pn1"> <asp:UpdatePanel ID="upCreateEdit1" UpdateMode="Conditional" runat="server"> <ContentTemplate> <label id="UserID" runat="server"> User ID</label> <asp:TextBox ID="txtUser_Id" Enabled="false" runat="server" MaxLength="10"></asp:TextBox> ...