How do I downgrade my iphone WITHOUT ITUNES?

I have a 3gs with iOS 4.2.1 that my mom gave me. I don't want to jailbreak it for another phone provider or anything. My old iphone crapped out and I just want to connect it to my itunes, but it won't since I have a powerbook g4 with Tiger, hence I can only get itunes 9, and the 3gs ios only works with itunes 10! the damn phone won't even sync or be recognized by itunes, so Im thinking there has the be some other program (not app) that'll I can use to change the iphone firmware without itunes. I've looked at all the Cydia stuff and it's daunting, I just want an alternate program that'll do the quick job that I need to it to do, downgrade my iphone to 3.1.3 or whatever. The apple store ain't no help at all, already been there.