The "1636 Squirrels" ROM is the required base for the "big three" of modern Pokémon ROM hacking:
What's the difference between different roms? : r/PokemonROMhacks 1636 - pokemon fire red -u--squirrels-.zip
The term "Squirrels" refers to the specific scene group or individual who originally dumped the Pokémon FireRed cartridge into a digital format (ROM). In the early days of ROM sharing, different dumps were numbered; the Squirrels dump was assigned the number in the GBA release database. The "1636 Squirrels" ROM is the required base