Yes, FIRTHS is a Scrabble word!

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

Is it a valid word?

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

English linguist who contributed to linguistic semantics and to prosodic phonology and who was noted for his insistence on studying both sound and meaning in context (1890-1960)
a long narrow estuary (especially in Scotland)

Also: j. r. firth, john rupert firth.