Ah, the classic Missingno. cheat. Well, the one you've heard is for the original Red and Blue/Green versions, this doesnt work on fire red.
However in the 3rd Gen, there is a variety of Missingno.'s. ONe looks like the empty Pokédex data for a poké you havent seen, the black ball with a question mark in it. The other looks like 2 pixellated question marks, which functions as a real pokemon and doesn't glitch your game. The pixellated ? Missingno, I have been hunting on my Sapphire ever since I heard of it, I've been searching for it's encounter code for Action Replay V.3 if anyone knows what it is?
Anyway, hope this helped :)