How to Put the iPhone 3G on DFU Mode
DFU Mode on an iPhone is a form of Recovery Mode. When put into DFU Mode, the iPhone does not load the on-board bootloader, which checks for firmware versions. When the iPhone is in typical Recovery Mode, the iBoot bootloader prevents you from downgrading it to an earlier firmware version. DFU Mode allows you to downgrade the firmware. Putting the iPhone into DFU Mode is a bit tricky, as it takes an exact timing of button pushing.
Connect the iPhone to your computer.
Open the iTunes application.
Press and hold the "Power" button (on the top of the phone) and the "Home" button (below the screen) simultaneously.
Release the "Power" button immediately after 10 seconds. Continue holding the "Home" button. If you have done this successfully, the iPhone's screen will remain black, and iTunes will notify you that it has detected an iPhone in "Recovery Mode." It may take several tries to get the timing exactly right.