The problem was like... the page get called twice!!! There is case when the server side code called twice, not client side.
After doing some good work found the solution, and was simpley good. The following line was causing the problem.
<asp:Image ID="imgTest" runat="Server" />
In first look you can see there is no problem with it. But there is still problem; the missing ImageUrl. Mozilla send request to current page asking for an image, which we havent set yet; as the value get assign from server side, by this reason the only server side code is get executed twice!!!!
Interesting, isn't it!!
So here is the solution:
<asp:Image ID="imgTest" runat="Server" ImageUrl="abc.jpg" />