Pokémon FireRed and LeafGreen/Mt. Ember

In this area you can catch the legendary bird at the top of the mountain and near the bottom you can get the Ruby plate which is necessary to complete the network machine and be able to trade with Ruby/Sapphire.

To get to Mt. Ember, you have to use SURF. It's just north of One Island, you have to travel on Kindle Road.

Kindle Road contains a number trainers and the Ember Spa where you will find

HM06 (Rock Smash). Along this route you can find, , , in the grass and  while surfing.