Save Frequently
Because Pokémon FireRed is played on a Game Boy, data can be easily lost unexpectedly when the batteries die. Thus, it's important to save your game at important crossroads: before you enter a gym, a villain's hideout or headquarters, and any cave, especially if you don't typically carry an Escape Rope. It's also beneficial if you lose a battle: then, you can simply restart your game and begin again at the point at which you saved.
Battle Everyone In The Gyms
It's not just important to beat every gym leader -- it's equally important to beat everyone along the way. Typically, there are a handful of other trainers in each gym, too. By battling these trainers, you'll get a sense of what type of Pokémon the gym leader will be using in your fight. This gives you time to prepare and bulk up before facing the final challenge. Trainers in gyms also tend to have a lot of cash on them, so you'll make decent money by winning these battles.
Stock Up At Poké Marts
There's nothing worse than getting caught in the heat of a battle without any potions or status healers left. Remember: The money you win in battles isn't just for Poké Balls; it's for keeping your Pokémon in top shape along your journey. Carry at least a dozen different potions on you at all times. Max Potions and Full Restores are ideal because they will completely heal your Pokémon, regardless of their HP, but Super Potions and Hyper Potions can be just as effective if used early in battle. Full Heals will get rid of any status problem your Pokémon may have, but they are pricey, which makes them difficult to have in stock. Buy a handful of Antidotes, Paralyze Heals and Awakenings, and you should be able to get by. Before you enter a cave or hideout, make sure you have at least one Escape Rope on you, too.