Web Forms :: Disable Button After Validation But Before Post Back

I have orders page. When user submit Order, I need to disable the button to avoid multiple transactions. I also have few validation controls on the page, so button should not be disabled untill page is about to be posted. I dont have any custom javascript for validation. It is just these asp.net validation controls. What is the best way to achieve that? I have spend about 2 hours on google and found various solutions, but nothing seems to work. Each has some issue. On button I add OnClientClick="return btnOnClick this, 'Payment');" Below is what gets called. Unfortunately if I disable button, it cancels post back. So I am calling dopostback and also adding few lines on server side to see which button caused postback. It works fine in IE8 and Chrome. But for Firefox on server side arg comes null. Any ideas what is wrong with my code? Or any other suggestions to do it in a better way?

C# Server side
string arg = this.Request.Params.Get("__EVENTTARGET");
if (arg == "Payment") { this.ProcessPayment(); }
Button
<asp:Button ID="btnSubmitPO" OnClientClick="return btnOnClick(this, 'Payment');" runat="server" Text="Submit Order" ValidationGroup="G" />
alert(group);