Pokemon FireRed and its sister game LeafGreen are remakes of the very first game Pokemon Red and Green. While the originals were Generation I, FireRed has some new features introduced from Ruby and Sapphire. FireRed still maintains the same story from the original but has some added story and more content.
It has new features like Pokemon Natures, Abilities, and more. The updated visuals and Pokemon from Gen II and III are always welcome if you like the originals.