Can you get a Ninetales in Pokemon FireRed version?

You cannot get a Ninetales directly in Pokémon FireRed. Ninetales is the evolved form of Vulpix, and Vulpix is exclusive to Pokémon LeafGreen.

Therefore, you'll need to trade with someone who has a Vulpix from Pokémon LeafGreen to obtain a Ninetales in your FireRed game.