There are 12 fireflies in Shantae. Getting all 12 will allow Shantae to read the scroll in the Firefly Shrine in Water Town that will allow her to learn the Healing Dance. They only appear at night, and only in outdoor locations (so there's no point in searching in caves or labyrinths).
Because - unlike other collectible items in the game - Fireflies are only useful when you have all of them, it's best to leave the Firefly hunt to the end when Shantae is more powerful and can transform into the Harpy or Tinkerbat. This will make the whole process easier and faster.
About two thirds of the way through the Snail area is a rocky area that can be climbed. Immediately to the right of the top of this plateau is a Firefly.
In the Hidden Waterfall area, stand on the branch on the far right and turn into either the monkey or the harpy and climb/fly upwards through the gap. There is a little chamber with a Firefly in it.
The leftmost rocky part of this area (the one that features a vertical shaft lined with spikes) has this Firefly at its summit. While it's possible to get it with the Monkey and the Spider, it's far easier just to come back later as a Harpy.
Close to the right entrance is a tall climbable surface for the Spider. Towards the top there is a platform with two bats nearby. From there, drop to a platform just out of sight below that one where you will find the Firefly.
It is found at the first of the walls that initially divides this area. If you use the Monkey to climb up, you will find a little area inset into the wall with a Firefly.
