Trying to close the page that was open with the same statement

I have a page "NonClosable.aspx". This page I opened successfully by the following statement:
ScriptManager.RegisterStartupScript(Page, GetType(Page), "OpenWindow", "window.open('NonClosable.aspx')", True) On this page I put the following statement in Closing button_click event but its not responding as follow:
ScriptManager.RegisterStartupScript(Page, GetType(Page), "CloseWindow", "window.close()", True) but its nothing is closed and no errors generted.
I am using FireFox. So why its not responding in case of the closing instruction??