For those folks who have upgraded to firmware 3.1.3 and tried to jailbreak their iPhones or iPod touch with redsn0w 0.9.4 but could not do so for any reason or are stuck and want to restore their older firmware 3.1.2, here are the high-speed direct download links. Let us know if you are facing any problem after jailbreaking your iPhone 2G and 3G or iPod touch 2G and 3G (non MC older models)

Download iOS/iPhone firmware for iPhone (2G, 3G, 3Gs and 4), iPod Touch (2G and 3G) and iPad

Important

Now you can do untethered Jailbreak of your iPhone 3G/3Gs running latest firmware 3.1.3 with "Spirit".

Step by Step iPhone and iPod Touch (All models) Firmware 3.1.3 Jailbreak Guide Using Spirit

And here is the process for restoring your to the older firmware 3.1.2 in simple two steps:

Step 1: Connect your iPhone to PC/Mac and open iTunes

Step 2: Click on the device and then press Shift + Left Click. This will open a window asking for the firmware location you want to restore. Give it the location where you downloaded the firmware and click OK and you’re done!
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Update 1:

Apple has stopped signing the firmware 3.1.2. See the link to know what it implies for your iPhone.