I actually figured this out with Bob a while back, but just to reiterate, the following code can get client side validation controls to work correctly in Firefox (or you can always do server side validation at the expense of postbacks):

C#

Page.Validate();

if ( !Page.IsValid )
{
return;
}
//Continue Your code

VB.NET

Page.Validate()

If Not Page.IsValid Then
Return
End If

'Continue Your code

Via Sushila Patel.