This stage unlocks in Act 2. You need to obtain every firefly in one run, and it's easier then it sounds. Not counting the platform you start on, grab the four on the upcoming platform, then grab the four IN the platform using pipe bombs and fans, then the fireflies start going upwards, there's two sets of two before you reach another platform when it follows the pattern of platform, two, platform with two IN it, two, and repeat. There are six platforms in the sky, and the last two just have four fireflies on the top. Just make sure you move fast to get the pieces you need.
If you are having issues with the fireflies on this level use the following map:
Every Kite Soars Firefly Map