measured my 18" 94 Explosif.
when compared to that and taking into account how Kona measured their bikes (seattube middle to top) I would say your Fire Mountain seems to be a 20" which is wee to big for me (178 cm tall)
What is the most challenging: to find a fork with a steerer tube fitting the 6" headtube weather f.e. the 18" Explosif has a 4" steerertube.
so the search continues - sorry would have liked to take the frame from you.