The firie spelling has more support from dictionaries, being listed in the online Oxford and Collins dictionaries, and in Wiktionary. Firey is listed as an alternate spelling by Wiktionary, but seems not to be in any other dictionary. But in usage it seems that firey is the version more frequently written. For example the Age has 32 hits for firie and 78 for firey.
(Exact comparisons are not 100% reliable because both variants occasionally appear as a misspelling of the adjective fiery. This even in professionally edited publications, buttressed by paywalls. Thus, the most recent example of firey is in today's horoscope, where Leos are informed that, "Firey, enthusiastic Mars is infusing your sector of work and industry with a sense of fun, productivity and proficiency." It's enough to undermine one's confidence in astrology!)
Anyhow, I'll add both spellings as rare words.