Okay, the last person was an idiot. I don't think you can, but Fire Red and Leaf Green did not come out before Ruby and Sapphire.
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You can do this after you find the Ruby and the Sapphire for Celio in Pokemon FireRed. then you can trade with Ruby, Sapphire and Emerald.
Ruby/Sapphire/Leaf Green/Fire Red/and i think Emerald. if not, trade from emerald to Ruby/Sapphire, and from ruby/sapphire to leaf green. You need to have the Sapphire and Ruby before you trade. you get these after the elite four.
trade it from ruby or sapphire.
You can't. You can only get jirachi in ruby and sapphire. You would have to trade from ruby or sapphire.
If you want to trade to ruby or Sapphire you have to fine the ruby and the Sapphire on the seivi islands. If you just want to trade to leafgreen then you can trade at any time.