firstly

As an adjective primary

is the first in a group or series.

As a noun primary

is a primary election; a preliminary election to select a political candidate of a political party.

As a verb primary

is (us|intransitive) to take part in a primary election.

As an adverb firstly is

(formal) in the first place; before anything else; first.

As adverbs the difference between preferentially and firstly

is that preferentially is in a preferential manner while firstly is (formal) in the first place; before anything else; first.

As an adverb firstly is

(formal) in the first place; before anything else; first.

As adverbs the difference between firstly and secondly

is that firstly is (formal) in the first place; before anything else; first while secondly is in the second place.

As an adverb firstly is

(formal) in the first place; before anything else; first.