You can't get any of the Hoenn starters Sceptile (Treecko), Blaziken (Torchic), or Swampert (Mudkip) in FireRed/LeafGreen; you'd need to trade from Ruby/Sapphire/Emerald.
If you do trade a Sceptile, Blaziken, or Swampert (or one of its previous forms) from RSE, you should probably mate that Pokémon with a Ditto in your RSE game first. After you hatch the egg in Ruby/Sapphire/Emerald, you can either trade that baby starter or the original higher-level Pokémon to your FireRed/LeafGreen.
That way, you have one form of the starter in one game and another form of the same starter in the other game.
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In emerald and ruby, sapphire is Blaziken. D and P is Giratina FireRed and LeafGreen is Tyranitar
No you cannot physically get Swampert in FireRed instead you will have to trade for one.
Firered and leafgreen both can catch different Pokemon but there pokedex is the same, here is some examples: magmar only in leafgreen not firered, oddish in firered not leafgreen, ekans in firered not leafgreen, bellsprout in leafgreen not firered, pinsir in leafgreen not firered.
Bellsprout has to be found wild, in leafgreen, you need to trade from Leafgreen for Pokemon.
You can not catch it in leafgreen,but in firered you can.