first off vs foremost

first off is not an adjective while foremost is an adjective.

Word	Noun	Adjective	Verb	Adverb
first off	No	No	No	Yes
foremost	No	Yes	No	Yes

As adverbs, first off and foremost are synonyms defined as:

Other synonyms of first off include first, first of all, firstly.

first off (adjective)	foremost (adjective)
	ranking above all others
	situated closest to the bow
	preceding all others in spatial position

first off (adverb)	foremost (adverb)
before anything else	prominently forward
	before anything else