Can you go to the Johto region in Pokemon FireRed?

Answer 1

You can the Pokemon from the johto region once you have the national pokedex. To get the national pokedex, catch 60 or more Pokemon from the kanto region then go and see prof. Oak in his lab in Pallet town.
You can't.... But you can get johoto Pokemon in the game and from Pokemon colluseam.

Answer 2

You can't go to Johto, but you can catch Johto pokémon. Obtain the National Pokedex (defeat Elite Four and catch 60 species of Pokémon to obtain the National Pokedex) then the Sevii Islands are the place to go.