The headline feature is right in the title. You can catch all 251 Pokémon from the first two generations (the original Kanto Dex and Gen II Johto additions). This effectively combines the exclusive Pokémon from both FireRed and LeafGreen, eliminating the need for trades to fill your Pokédex.
Fire Red alone only contains the original 151. To get the remaining 100 Johto Pokémon, you cannot simply walk into tall grass. You must use the post-game. pokemon fire red 251
You cannot hit the 251 milestone using a single copy of FireRed. The game enforces structural limitations that require external connectivity. LeafGreen Exclusives The headline feature is right in the title