Where in fire red is the guy who will trade a Jynxfor poliwhirl?

You're likely thinking of the trainer in Fuchsia City, located on the left side of the city as you enter from the south. This trainer is named "Mr. Psychic" and he's the one who offers a Jynx for your Poliwhirl.

He's a bit of a tough opponent, so make sure your Poliwhirl is adequately leveled up before trading!