@seigeweapon: IIRC, the first FW version that handled those models is 5.4.3.2 (at least mine did, that's what it shipped with).
Since that's a public, full release, you shouldn't have any problem getting your hands on it and flashing it.
Those are Kindles, the usual caveat of "don't flash anything lower than what that specific model was originally shipped with" *always* applies

.