firebox vs furnace

firebox and furnace both are nouns.

Word	Noun	Adjective	Verb	Adverb
firebox	Yes	No	No	No
furnace	Yes	No	No	No

As nouns, furnace is a hypernym of firebox; that is, furnace is a word with a broader meaning than firebox:

firebox: a furnace (as on a steam locomotive) in which fuel is burned
furnace: an enclosed chamber in which heat is produced to heat buildings, destroy refuse, smelt or refine ores, etc.

firebox (noun)	furnace (noun)
a furnace (as on a steam locomotive) in which fuel is burned	an enclosed chamber in which heat is produced to heat buildings, destroy refuse, smelt or refine ores, etc.