Top 10 Differences Between Pokemon Red/Blue and Pokemon FireRed/LeafGreen

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 get 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 travelling 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.

Holy crap... never knew that

In Pokemon Red, there were no Steel nor Dark types. This changed since generation 2. In Pokemon FireRed, there's Steel and Dark type moves/Pokemon.

Let's compare both Red and FireRed:
Bulbasaur: It learns Vine Whip at level 13 in Red and in FireRed at level 10. As an Ivysaur, it learns Razor Leaf at level 22 in FireRed while in Red 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 being 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 a 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 ...more

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 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 than with the critical hit. Razor Leaf is super strong against Water, Rock and Ground type Pokemon, but just pretty average against most of the 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 it's Flying type. In the later games, Charizard doesn't resist nor is weak to it. And Fire types resist Ice type moves after generation 1.

In FireRed, you go to the seven 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 colored and well detailed. In Pokemon Red, it's in black and white and it's not very easy too see. At least you can see well in FireRed.