I still have my old iPhone 3gs (AT&T) with iOS 6. I have a back up of it as well. I also have a iPhone 4s (Sprint) with iOS 7.
Can I grab the ipsw file off my iPhone 3gs and install it on my iPhone 4s?
I will not install any ipsw from the net. It is not worth the risk of getting an infected or spyware file.