"First" is a flat adverb meaning it takes the same form as its adjective pair, but while some flat adverbs mean the same as their "-ly" versions (the sun shone bright/brightly), "first" and "firstly" have diverged slightly and are not always interchangeable.

Is first a verb or adverb?

first (adverb) first (noun) first (pronoun) first–class (adjective)

Is first an adjective of number?

Adjectives of number refer to things that can be counted-even if it is an indefinite amount in the sentence. ... Definite Adjectives of Number. These are numbers and amounts, both cardinal numbers and ordinal numbers: one, thousand, gallon, first, fifth, etc.