Version Exclusive Pokemon
The most significant difference between "Pokemon FireRed" and "Pokemon LeafGreen" is the version of exclusive Pokemon found in each game. "Pokemon FireRed" has Oddish, Elekid, Psyduck, Ekans, Growlithe, Scyther, Shellder, Wooper, Murkow, Skarmory, Qwilfish, Delibird and their evolved forms as the version-exclusive Pokemon. "Pokemon LeafGreen" has Bellsprout, Magby, Slowpoke, Sandshrew, Vulpix, Pinsir, Staryu, Azurill, Misdreavus, Sneasel, Remoraid, Mantine and their evolved forms as the version-exclusives.
Deoxys
Deoxys is a Pokemon that can be caught only by getting an Aurora Ticket, a promotional item given out at Nintendo events. Deoxys does not evolve into another form, but there are four distinct versions of this Pokemon. Each version has a unique appearance and stats based on which version is being used. "Pokemon FireRed" players will find the attack form Deoxys, which has high attack, average speed and low defense stats. "Pokemon LeafGreen" players will find the defense form Deoxys, which has high defense, but low attack and speed stats.
Minor Differences
There are two additional minor differences between "Pokemon FireRed" and "Pokemon LeafGreen" that have almost no impact on the player. The flavor text for each Pokemon is different between the two versions. The flavor text line usually states the same fact about the Pokemon, but the wording differs slightly. The rarity of Nidoran (F) and Nidoran (M) also differs based on which version is being played. Nidoran (F) is common to encounter in "Pokemon FireRed" but rare in "Pokemon LeafGreen." Nidoran (M) is rare to encounter in "Pokemon FireRed" but common in "Pokemon LeafGreen." Capturing both of these Pokemon will take approximately the same amount of time no matter which version is being played.