To get to this hideout search the northeastern section of the first area of World 6 for a green pipe. This will be right next to World 6-2: Spooky Seasick Wreck, on a little raised section. The third hideout for the Fire Bros will contain a few of their cousins, in addition to the Fire Bros themselves. There will just be a single Fire Bro, two Hammer Bros, and two Boomerang Bros.
Naturally, this is a lot of enemies at once, especially with them shooting out projectiles and jumping around. If you have a ranged power-up, this makes things a lot easier, since you can stand back to dodge their projectiles and counter with your own. Remember that the hammers are in an arc, the boomerangs will come back, and there’s nothing special about the fireballs. Also, since some of them are in close proximity to one another, their own projectiles can damage one another. Collect the Green Star when all enemies are gone to finish.