All 3GS with 3.0 have the old bootrom.
To OP, use Redsn0w to pwned(accepts custom firmware) JB the 3.0 firmware. Afterward, from now onward, if want to upgrade the firmware, create or search (google) for custom JB firmware (one that keeps the very stable 4.26 baseband). Then, restore with that custom firmware through ITunes.