ensimmäisten
Ensimmäisten is a Finnish word form that functions as the genitive plural of the adjective ensimmäinen, meaning “first.” It is used to express possession or a relationship to a subset of a larger group, translating roughly as “of the first [ones].” In Finnish, adjectives decline to agree with the noun they modify in number and case, so ensimmäisten appears before a head noun in a genitive plural context.
Usage of ensimmäisten typically occurs in phrases that identify a subset or a position within a group.
Grammatical notes and etymology: ensimmäisten is formed from the stem of ensimmäinen with the genitive plural
In sum, ensimmäisten is a specific inflected form used in Finnish to denote “of the first [ones]”