How to undrestand why firefox stop post back?

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ASP.NET

Gridview

I have a grid view that there are some rows in it that contain path of images in a folder.
In grid view selectedIndexChanged when click in each row, address of image is created and i show it in a image.

int x = GridView4.SelectedIndex;
string y = GridView4.DataKeys[GridView4.SelectedIndex].Values["attachName"].ToString();
y = "~\\uploads\\"+EnvID_hf.Value + "\\" + y;
Image2.ImageUrl = y;        

it works for the first time when i click the gridview but after that nothing will happen in the page and if i click again on gridview there is no postback, and it seems to be stoped for working.
Of course it does not happen in IE but i want to use firefox and I have this problem.
Thanks in Advanced.