by the term of goa olaka
Its impossible to capture any hoenn Pokemon in firered so you have to trade hoenn Pokemon from ruby, sapphire and emerald to get them.
You can only have Pokemon from Hoenn to Jhoto ONLY if you trade them.
From what I have heard, Regigigas is a Hoenn pokemon. You will have to trade it from Platnium. Sorry!
You can't get Hoenn Pokémon in Leaf Green unless you trade from Ruby/Sapphire/Emerald.
To complete hoenn pokedex you do not need JIRACHI & DEOXYS at all.you can't get all Pokemon on Pokemon emerald so TRADE or CHEAT !!
you can't catch any hoenn Pokemon in firered or leafgreen you must trade hoenn Pokemon from ruby or sapphire or emerald to leafgreen or firered.
Catch trade n evolve Pokemon.
You can't because Pokemon gold, silver and crystal, which are in johto are on gameboy colour and Pokemon ruby, sapphire and emerald, which are in hoenn are on gameboy advance. Pokemon shiny gold, which is also in johto is on gameboy advance but it isn't an official Pokemon video game.
You can't go to Hoenn on leafgreen, but after you beat the elite four you can trade with hoenn and go to the sevii islands. (all seven of them)
you cant unless u trade
short answer: no
Its impossible to catch any hoenn Pokemon in firered if you want hoenn Pokemon like groudon and kyogre trade them from ruby, sapphire or emerald to receive them in firered.