This is a simple Minetest mod by Shara RedCat that adds fireflies to the world on mapgen, which can then be caught in a net and placed in bottles to provide light. Note that they only appear on newly generated map.
This mod was originally adapted from original but with featured ethereal more biomes, that was adapted from the Hidden Worlds subgame to work with Minetest Game, and has since been added to Minetest Game itself. This version (at https://github.com/Ezhh/fireflies) is now maintained independently from the main Minetest Game version for the purpose of possible expansions to the mod which may or may not also be added to Minetest Game at a later time.
Please note that the current stable version of Minetest (0.4.16) is unable to position the fireflies above groundlevel. The mod is otherwise fully functional and can be used; but requires the current dev version of Minetest for optimal firefly placement.