Please note that fireflies only appear on the map during initial mapgen, so if adding this mod to an existing map they will only appear in areas that have not previously been visited.
Fireflies was created for use with the Hidden Worlds subgame which I'm currently working on. This mod is an adapted, MTG compatible, version which will be maintained separately and may or may not end up with all the same features.
In game:

UPDATE: The fireflies mod has now been added to Minetest Game. I will continue to maintain this version of the mod separately in order to test adjustments that may (or may not) become later additions to the Minetest Game version.
Download: https://github.com/Ezhh/fireflies/archive/master.zip
Source: https://github.com/Ezhh/fireflies
Compatibility: This mod was written with current dev, not 0.4.16 stable in mind. It will work and be fully functional in 0.4.16 stable; however, the fireflies will only spawn at one height instead of at varied heights.
Depends on: default, vessels
License: The code for this mod is released under MIT (https://opensource.org/licenses/MIT). See license.txt in the mod for texture licenses.