on fire vs afire

on fire and afire both are adjectives.

Word	Noun	Adjective	Verb	Adverb
on fire	No	Yes	No	No
afire	No	Yes	No	No

As adjectives, on fire and afire are synonyms defined as:

Other synonyms of on fire include ablaze, aflame, aflare, alight.

on fire (adjective)	afire (adjective)
lighted up by or as by fire or flame	lighted up by or as by fire or flame