foremost vs firstly

foremost is an adjective but firstly is not an adjective.

Word	Noun	Adjective	Verb	Adverb
foremost	No	Yes	No	Yes
firstly	No	No	No	Yes

As adverbs, foremost and firstly are synonyms defined as:

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

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

foremost (adverb)	firstly (adverb)
prominently forward	before anything else
before anything else