Firmware of an iPhone is the set of software that enables the device to run smoothly. The hardware of the iPhone is able to function properly with the help of this firmware. This firmware sits between the hardware and the software and fixes bugs and makes functioning of the features of the phone smoothly. If you are making use of Phone 3GS and have achieved jailbreak for your device, you will require to update the operating system whenever there is an upgrade announced by the company. Download iPhone 3G iOS 4 2 1 custom firmware free here to upgrade you iOS.