Fireflies, a mob that only consists of two pixels, were announced to be added in the Wild Update. Besides looking nice in the new mangrove swamp biome, the primary use for fireflies was to be eaten by frogs. Mojang then found out that eating fireflies is harmful to frogs in real life, and they didn't want to encourage players to go on a frog poisoning rampage. As of now it is unclear whether fireflies will still be added to the game or not. By downloading this add-on, however, you can still use fireflies in your game! While they cannot be eaten by frogs, they can still add ambiance to your worlds.
Fireflies will spawn naturally in forest biomes and in swamps. When spawned naturally they will only be visible at night, and they will despawn in the day. Fireflies can be caught in an empty bottle, although it can be tricky. Each little firefly is not a separate entity. This means that to catch them you must interact at the center of the swarm of fireflies. Once you have obtained a bottle of fireflies, you can craft a firefly lantern. The crafting recipe is the same as a normal lantern, but instead of a torch you must use the bottle of fireflies.
If the fireflies ever get too annoying for some reason, there are several ways to dispose of them.
You can catch them in a bottle, kill them by "breaking" the center, spawn a bat and let it eat them, or just delete your world because you never want to see them again.
(I don't recommend the last option)
When catching or killing fireflies, they will not disappear immediately. It will take a couple seconds for all the bugs to despawn.