You don't need a badge to get strength but you need a badge to use strength and that badge is the rainbowbadge and that is the 4th one in celadon city.
Chat with our AI personalities
The marsh badge is in the saffron city gym you must beat Sabrina the gym leader to get it, but to do that you must beat team rocket in the silph co building.
go to saffron city to beat Sabrina to get the 6th badge however you need to first go through silph co to do this.
beat the league
Beat the Gym Leader
To get the fifth badge in Pokémon FireRed and LeafGreen version you have to beat Koga. He uses poison type Pokémon.