This one's gonna take a while. First here's what you need (aside from Firered and Black) :
1. 2 DS systems (At least one with Gameboy compatability)
2. 1 Diamond, Pearl, Platnium, Heartgold, or Soulsilver game with Pal Park ready to be used
3. Be at the Poketransfer Lab on Route 15 in Black (You have to have beaten the main story to go there)
4. Patience
First, insert Firered into a DS system with a Gameboy slot with any game in step 2. Then, select what Pokemon you want to transfer. After that, use the Pal Park feature to transfer them to that game. It may take a while to catch them all, but it is needed. Next, make sure that the Pokemon from Firered to transfer know NO HMs from that game and are in one of your PC Boxes in the PC. Then, turn off that DS, turn it on again, and activate DS download play (Keep the game in the system). Then, get the other system with Pokemon Black and go inside the Poketransfer lab, talk to the scientist in the back, and say yes and save. Then look on the other DS and wait to see the option for the Poketransfer. Tap it when it comes up, and use that DS. The PC boxes from the game in that system will come up. Choose the ones that are from Firered and hit yes. Then you have to do the transfer game, but if you win, they'll be on Black. Repeat until all from Firered are transfered. Good luck!
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You'll have to transfer it into Pokemon Black or trade for it.
Pokemon Red/Blue/Green: it is available as a starter. Pokemon Yellow: go to Vermilion City and talk to Officer Jenny in the middle of the city. Pokemon Gold/Silver/Crystal: you will need to trade for it from Pokemon Red/Blue/Yellow. Pokemon Sapphire/Ruby: you will need to trade for it from Pokemon FireRed/LeafGreen. Pokemon FireRed/LeafGreen: it is available as a starter. Pokemon Diamond/Pearl: you will need to transfer it from Pokemon Sapphire, Ruby, Emerald, FireRed, or LeafGreen. Pokemon Platinum: you will need to transfer it from Pokemon Sapphire, Ruby, Emerald, FireRed, or LeafGreen. Pokemon HeartGold: you can pick it from Prof. Oak in Pallet Town. Pokemon Soulsilver: you can pick it from Prof. Oak in Pallet Town. Pokemon Black(2)/White(2): transfer, Dream World, or trading is required. Pokemon X/Y: it can be received from Prof. Sycamore in Lumiose City. Pokemon Omega Ruby/Alpha Sapphire: you must trade from X or Y.
You go to pal park where you can round up your migrated pokemons
put it in your gba spot on your ds. then turn on the ds. when you go to the main menu press migrate from firered. choose yes then yes and pick your Pokemon. then go to pal park then done note: you can't return your Pokemon back to firered so be careful
In Pokemon firered you can't, but in Pokemon leafgreen you can get it then trade it to Pokemon firered.