at first hand vs firsthand

at first hand is not an adjective while firsthand is an adjective.

Word	Noun	Adjective	Verb	Adverb
at first hand	No	No	No	Yes
firsthand	No	Yes	No	Yes

As adverbs, at first hand and firsthand are synonyms defined as:

at first hand (adjective)	firsthand (adjective)
	received directly from a source

at first hand (adverb)	firsthand (adverb)
from the original source; directly	from the original source; directly