Iphone 3g version 4.2.1 modem firmwear 05.15.04

I have an Iphone 3g FW 4.2.1 Baseband 05.15.04 used to be vodaphone pay and go i need it on orange contract
And I want to jailbreak and unlock it
2. do i need to downgrade to a lower firmware that might have a stable and proven jailbreak and unlocking? what would be the step by step process?
Help greatly appreciated!
angel