To get the sixth badge in Pokemon FireRed and LeafGreen, you will need a Charmeleon that knows Flamethrower, or a Wartortle that knows Surf. Next, you should have a Graveler that knows Magnitude, for the Ground-Type is stronger than the Poison. Also, bring a lot of Antidotes or Full Heals, just in case your Pokemon gets poisoned. It's also beneficial if you have a Psychic or Steel-Type Pokemon, since they're both good against Poison too. Poison-Type attacks won't affect Steel-Type Pokemon at all!
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Fortree City ( Feather Badge ) Leader's pokemon: Altaria, Swellow, Skarmory, Pelipper
The sixth gym badge (the Marsh badge) is located at Saffron City's gym. It is in Gym Leader Sabrina's possession, so you have to defeat her to obtain it.
in gym durr
i haht Pokemon soulsilver so live with it itiet
Back in the 3rd town where your dad is hope this helps :)