in pokemon fire red on island 5 how do i get in the rocket warehouse?

Answer 1

You'll need to complete the sapphire mission

The sapphire is found in the bottom of the cave on island six. You'll need to head inside (use cut on the door) and fall into the holes in the correct order (one time each, four total times) and then you'll be met by a scientist at the bottom. He'll steal the sapphire plate and give you the password to the team rocket hideout on island 5. Head there and battle through and after beating the boss here, head north and confront the scientist in battle. Once you defeat him he'll hand over the sapphire plate.