Top 10 Differences Between Pokemon Red/Blue and Pokemon FireRed/LeafGreen
Pokemon Red/Blue and Pokemon FireRed/LeafGreen are almost the same, but there are many differences. Red/Blue is generation 1 and FireRed/LeafGreen is generation 3. Pokemon FireRed is basically an updated version of Red with less glitches, more easy to play and have more to do than the original version. I'll only compare Pokemon Red and FireRed because they're better than Blue and LeafGreen. Not gonna mention the other editions. However Blue is like Red, but with a very few differences (mostly wild Pokemon) and LeafGreen is like FireRed. Now, let's compare between the original Red version and the updated Game Boy Advance version, FireRed.Charizard can't learn any Flying type moves in Red. If you're planning to have a Flying type Pokemon in Red and you decided to choose Charmander, you're definitely out of luck. It basically means you have to go out of your way to find another Flying type Pokemon like Pidgey or Spearow.
In FireRed, Charizard can learn Flying type moves. It learns Wing Attack at Level 36 after it evolved into Charizard. It's definitely a big deal. It destroys Fighting type Pokemon very easily.
It can also learn the move Fly if you decide to teach it. Fly is useful, and you can go anywhere in the game with it, besides places you've never been to without traveling first.
If you're planning to have a good Flying type Pokemon in FireRed, I'd recommend Charizard. The other Flying Pokemon are a bit too weak.
In Pokemon Red, there were no Steel nor Dark types. This changed since Generation 2. In Pokemon FireRed, there are Steel and Dark-type moves and Pokemon.
Let's compare both Red and FireRed:
Bulbasaur: It learns Vine Whip at level 13 in Red and at level 10 in FireRed. As an Ivysaur, it learns Razor Leaf at level 22 in FireRed, while in Red, it is at level 30. In FireRed, it can learn Sleep Powder at level 15 as a Bulbasaur, while in Red, it will take forever, like even after becoming a Venusaur.
Charmander: It learns Ember at level 9 in Red, while in FireRed, it learns Ember at level 7, which is the ideal level to learn the first Fire type move. In FireRed, Charmeleon learns Flamethrower at level 34, just a bit before evolving into Charizard. In Red, it learns quite after it evolved into Charizard at level 46. Charmander also learns Metal Claw at level 13 in FireRed. Back in Red, Steel moves did not exist yet. Metal Claw also helps take down the first gym.
Squirtle: It learns Bubble at level 8 in Red, but in FireRed, it learns at level 7. In FireRed, it can learn Water Gun at level 13. In Red, it learns at level 15. As a Wartortle, it learns Bite at level 24 in Red, but at level 19 in FireRed. Bite used to be a Normal type move in Red. It became a Dark type move since Generation 2. FireRed is Generation 3. Bite also helps take down Psychic and Ghost type Pokemon.
In Pokemon Red, Psychic-type Pokemon were immune to Ghost-type Pokemon since they had no effect against them. In FireRed, Psychic-type Pokemon are weak to Ghost-type Pokemon. They fixed it after Generation 1.
Those moves are Slash and Razor Leaf. That was some sort of bug in generation 1, but it later got fixed. Still, those moves were extremely powerful. Without critical hits, they're still decent.
Slash is still quite a strong move overall, but not nearly as much as with the critical hit. Razor Leaf is super strong against Water, Rock, and Ground type Pokemon, but just pretty average against most other types besides Flying, Fire, Bug, Poison, Dragon, and Steel type Pokemon, which all resist Grass moves.
Since Fire-type Pokemon didn't resist Ice-type moves at that time, Charizard was weak to Ice types due to its Flying type. In the later games, Charizard neither resists nor is weak to it. Fire types resist Ice-type moves after Generation 1.
In FireRed, you go to the Sevii Islands and you can do a lot more than in Red. You get to catch some Generation 2 Pokemon, do unusual missions, and fight even more trainers.
The VS Seeker didn't exist until generation 3. Pokemon Red is a generation 1 game, but FireRed is a generation 3 game. Also, the VS Seeker helps you grind more Pokemon faster and to help you have stronger Pokemon. It's recommended to use it if you chose Charmander as a starter, or any starter really. It's optional to have a VS Seeker.
The graphics in Pokemon FireRed are very colorful and well-detailed. In Pokemon Red, it's in black and white and it's not very easy to see. At least you can see well in FireRed.
Since Red is only a Generation 1 game, you can't find any Generation 2 Pokemon whatsoever. Generation 2 didn't exist at that time.
FireRed, on the other hand, is a Generation 3 game, so it's normal to include Generation 2 Pokemon in it.