Yes, FIREWALL is a Scrabble word!

Letter values in Scrabble US: F 4, I 1, R 1, E 1, W 4, A 1, L 1, L 1.

Is it a valid word?

FIREWALL is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

(colloquial) the application of maximum thrust
(computing) a security system consisting of a combination of hardware and software that limits the exposure of a computer or computer network to attack from crackers; commonly used on local area networks that are connected to the internet
a fireproof (or fire-resistant) wall designed to prevent the spread of fire through a building or a vehicle