Comment 1 for bug 309971

Regardless of the technical need for this, I doubt that users interface will work as you wish. Most users won't understand the message, nor will they bother to learn what the dialog is trying to tell them. They will just think it's getting in their way of downloading files and be gone with it.

For further discussions about users and trying to forcefully make users make decisions about things they can't be expected to care about, see these well-written articles:

http://blogs.msdn.com/oldnewthing/archive/2003/09/01/54734.aspx
http://www.joelonsoftware.com/uibook/chapters/fog0000000062.html