In Fire Bar Cliffs, you'll find stairs with two fire bars. A Buzzy Beatle will be descending them. If you time it right, Mario can jump on that enemy and cause his shell to bounce repeatedly against the stairs, racking up extra lives the same general way that you could at the end of World 3-1 in the original Super Mario Bros. Keep in mind that there are numerous other places throughout the game where you can perform a similar trick, including the Tilted Tunnel level in Acorn Plains and the Switchback Hill level in Meringue Clouds.