headlong vs headfirst

headlong and headfirst both are adjectives.

Word	Noun	Adjective	Verb	Adverb
headlong	No	Yes	No	Yes
headfirst	No	Yes	No	Yes

As adjectives, headlong and headfirst are synonyms defined as:

As adverbs, headlong and headfirst are synonyms defined as:

headlong (adjective)	headfirst (adjective)
with the head foremost	with the head foremost
excessively quick

headlong (adverb)	headfirst (adverb)
with the head before the body	with the head before the body
in a hasty and foolhardy manner
at breakneck speed