I have a text box here in ASP page,

but when I press enter to have more extra lines, the form would actually submit. This only happens in Firefox. It's fine on IE.

Does anyone know how to fix that?

<asp:TextBox ID="txbMessage" runat="server" MaxLength="2000" width="240px" Rows="10" TextMode="MultiLine" />

Are you 100% it's textmode?

On the button click, add this code:

if(Page.IsValid)
{
     // put your code to handle the click
}