Downgrade/Restore iPhone 3G to iOS 3.1.3 or lower

Mac friendly instruction
http://www.iphonehacks.com/2010/07/how-to-downgrade-iphone-3g-from-ios-4-to-iphone-os-3-1-3.html

Windows friendly instruction along with Mac
http://www.techradar.com/news/phone-and-communications/mobile-phones/how-to-downgrade-iphone-ios-4-to-os-3-1-3-704705

Where are iPhone firmware files stored on my computer?
http://www.iphonefaq.org/archives/97320

RecBoot but…
http://jaxov.com/2010/07/download-recboot-fix-itunes-1015-restore-error/

RecBoot needs libusb.. so
http://jaxov.com/2010/08/install-libusb-windows-7-vista-64-bit/

As recommendation to Tinyumbrella, use that one..
http://jaxov.com/2010/10/use-tinyumbrella-to-kick-iphone-out-of-recovery-mode/

Donwload tinyumbrella here
http://thefirmwareumbrella.blogspot.com/

Here are list of iOS firmwares
http://www.iclarified.com/entry/index.php?enid=750