Yes, FARTHER is a Scrabble word!

Letter values in Scrabble US: F 4, A 1, R 1, T 1, H 4, E 1, R 1.

Is it a valid word?

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

to or at a greater extent or degree or a more advanced stage (`further' is used more often than `farther' in this abstract sense)
to or at a greater distance in time or space (`farther' is used more frequently than `further' in this physical sense)
to a considerable degree; very much
at or to or from a great distance in space
at or to a certain point or degree
remote in time
to an advanced stage or point