firewater vs John Barleycorn

firewater and John Barleycorn both are nouns.

Word	Noun	Adjective	Verb	Adverb
firewater	Yes	No	No	No
John Barleycorn	Yes	No	No	No

As nouns, John Barleycorn is a hypernym of firewater; that is, John Barleycorn is a word with a broader meaning than firewater:

firewater: any strong spirits (such as strong whisky or rum)
John Barleycorn: an alcoholic beverage that is distilled rather than fermented

Other hypernyms of firewater include booze, hard drink, hard liquor, liquor, spirits, strong drink.

firewater (noun)	John Barleycorn (noun)
any strong spirits (such as strong whisky or rum)	an alcoholic beverage that is distilled rather than fermented