There is a green pipe to the left side of World 5-5: Bob-ombs Below, which will take you to an underground area with this enemy encounter. When you enter this little battle, you will see two pillars here, as well as a Goomba Tower. You’re probably wondering where the Fire Bro is located, since this is a Fire Bro Hideout and all. Well, the Fire Bro (as you might have guessed) is at the top of the Goomba Tower. Your goal here is to defeat only the Fire Bro, so the Goombas don’t need to be stomped on at all.
Depending on what you have available to you, this might take a little longer than normal. Should you have a power-up, like a Cat Suit or Fire Flower, you can cut the Goomba Tower down to size. If you don’t have a power-up available, then you will need to utilize the moving towers on either side. Wait for one to get low enough for you to jump on them, then when it gets high enough, you can jump on the Fire Bro at the top of the Goomba Tower. However you do it, collect the Green Star that appears to finish.