Yes, FIRSTS is a Scrabble word!

Letter values in Scrabble US: F 4, I 1, R 1, S 1, T 1, S 1.

Is it a valid word?

FIRSTS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the first or highest in an ordering or series
the first element in a countable series
the time at which something is supposed to begin
the fielding position of the player on a baseball team who is stationed at first of the bases in the infield (counting counterclockwise from home plate)
an honours degree of the highest class
the lowest forward gear ratio in the gear box of a motor vehicle; used to start a car moving

Also: number one, number 1, get-go, starting time, first base, first-class honours degree, first gear, low gear.