The differences are little more than some cosmetic changes on the slide stop, and the safety levers. Also, the safety levers on first models were secured each other with a screw. The rear sight was secured with a spring and plunger on the "old model", later was secured with a torx screw and plastic washer............. there are some other minimal differences that I don't remember right now.
In fact is more accurate to talk about first production examples, and later production, it's just an evolution of a design I think.
Mike, I think he has read that on the Numrich webpage, they supposedly have two different parts lists for the "old" and "new" model, altough probably they have no idea of wich part goes on wich variation, if it's true they have these early parts.
Jorge: That is undoubtedly the answer. Numrich probably purchased a batch of useless parts left as "dead inventory" because after only few hundred guns, the Firestar was extensively redesigned and many of the early parts could not be adapted. The slides and frames of the early version, for example, were quite different, though Star did salvage the frames by grinding off part of the trigger guard.
Something similar happened with the Star Model 110 .22 rifle, except that only a few prototypes were assembled before the project ran into irremediable trouble. Numrich bought all of those parts, too, and tried to market them in the USA as DIY "kits". Of course, if Star could not make them work satisfactorily, there was little prospect that a "kitchen table gunsmith" could succeed, either.