In the world of ROMs, "Squirrels" refers to a specific scene release group (often credited as "Independent" or "Squirrels") that dumped the game from the cartridge. When you see the file name 1635 - Pokemon Fire Red (U)(Squirrels).gba , you are looking at a clean, unmodified copy of the official US release.
: Features updated Generation 3 graphics and sound, replacing the monochromatic look of the original 1996 Red and Blue.
: Includes the Special/Physical split (though technically Gen 4, some modern hacks add this to the Squirrels base), Pokemon Abilities, and the VS Seeker for re-battling trainers. How to Use It for ROM Hacks FireRed hack: - Pokémon Gaia Version - Page 60