Pokémon FireRed and LeafGreen/Mt. Ember

In this area you can catch the legendary bird at the top of the mountain. To get to Mt. Ember, you have to use Surf. It's just north of One Island, but you'll have to go past Kindle Road.

Kindle Road contains a number trainers and the Ember Spa where you will find HM06 - Rock Smash. If you enter the middle pool within the Spa, it will heal all your Pokemon. Along Kindle Road you'll be able to find, , , and in the grass, and  while surfing.

Near the bottom of the mountain you can get the Ruby Gem which is necessary to complete the network machine and be able to trade with Ruby/Sapphire, but for now, the area is blocked by those two Team Rocket members. You'll have to come back later. For now, continue to climb up the mountain and enter the cave.

The cave is not very long, and once you exit you'll find yourself at the top of the mountain. You'll have to solve an easy puzzle with Strength to reach Moltres. Make sure to bring a good amount of Ultra Balls!

Once you are finished here, head to One Island and talk to Bill. It's time to return to Kanto, and tackle the final Gym at Viridian City!

Viridian Gym
From the dock, Fly to Viridian City. The old man previously blocking the entrance has moved, and informs you that the Gym Leader has finally returned. Time to earn your final badge!

Once you enter, you'll see a small maze filled with Gym trainers. Technically you only need to face two of these trainers to face the Gym Leader, one Cool Trainer and a Black Belt both north in the room, but you should really face them all and get all that delicious XP!

Once you finish your battles, you will face the Gym Leader... It's Giovanni, Team Rocket's own leader! It seems that after you defeated at Silph Co., he has returned to Viridian to regroup, and is ready for a rematch with you!

Giovanni will no longer call upon his Kangaskhan, and has decided to focus upon his Ground type Pokemon. Expect to face a difficult battle, as all know Earthquake, and all will benefit from STAB.

Both Nido's have a decent chance to Poison your Pokemon by their Poison Sting move, or through their Poison Point ability. Expect a particularly fast Dugtrio that will be particularly annoying with Mud-Slap, decreasing your accuracy. Both Rhyhorns are slow but can be a threat if they unleash Rock Blast on you.

Water Pokemon will work wonders in this battle. Grass Pokemon will be useful against Dugtrio and both Rhyhorns, but Nidoking and Nidoqueen's Poison type will nullify any advantage that Grass types would usually get.

Once you defeat Giovanni, he will recognize that he still has a lot to learn, and vows to one day return. He awards you the Earth Badge, which makes all Pokemon obey you regardless of level. He also awards you TM26 Earthquake, the most powerful Ground move in the game. You have finally completed the Gym Challenges!

Before you leave, stand on the exact spot where Giovanni stood, and use Itemfinder. Your final reward for beating Giovanni is the Macho Brace, a held item that will double any EV's that your Pokemon gets from battle, but halves its speed.

Now it is time to leave Viridian City, and head to your final challenge: The Elite Four!!!