My belief is that from 1993 onwards the frames were the same. For 1991-92, although the Lava Dome and the Cinder Cone both had double-butted main tubes, the fact that they say that the Cinder Cone had the lightest Kona double-butted tubing implies that it may have had slightly thinner gauge and slightly lighter than the Lava Dome. But even then the Lava Dome was at a clearly higher level than the Fire Mountain.
I don't think either bike had butted stays in those days, although right at the death the Lava Dome had db stays in 2003. Prior to that, the Kilauea and then the Caldera seem to have been the lowest models with db stays.
Incidentally, speaking as the owner of a 97 Kilauea and a 97 Lava Dome, I can testify that db stays do make a difference, not always reflected in the eBay valuations of the various frames.