Tiny Umbrella is a free Mac utility that allows you to restore the firmware in iPhone 3GS's.
The iPhone 3GS has a special bootrom that checks the firmware that iTunes uses to restore the device. The bootrom requires that iTunes modify the images located inside the IPSW file and add a special signature to each one of the images in the IPSW which is only available from Apple.
If you try to restore a firmware that Apple is no longer signing, you will receive an error from iTunes and you will not be able to restore that firmware version to your device. TinyUmbrella allows you to use the SHSH files if you have saved them previously.
Note that TinyUmbrella does not require a Jailbroken iPhone to restore the firmware. However, it is a bit complicated to configure although help is available on the developer website.
TinyUmbrella makes it possible for you to restore the firmware in the iPhone 3GS.