I have a iphone 3g that is jailbroken with redsn0w. I tried to save the shsh blobs with tinyumbrella and ifaith and no luck. ifaith would say unsupported device and tinyumbrella would say it could not find any shsh blobs. but if my fw is 4.2.1 and 5.15 why can't I save the blobs? and if i can't save the blobs how am i going to restore later on?

I know i can unlock it with redsn0w and updating fw to 6.15 and using cydia ultrasn0w but then would I never be able to restore it since I can't make a custom firmware?

Any help will be greatly appreciated!