So recently I made a forest for my adventure game, and I tried to add a fireflies effect
https://gyazo.com/0bd81e07015760c52bf40585416480f1
https://gyazo.com/bb5a32f81e5271510d24fef0fcc0385d
Any ideas on how to make this effect better and more natural?
I would personally make the fireflies move, instead of only going up. They could rotate, move backwards and forwards, left and right, etc.
I would also make them flicker, as fireflies do flicker in real life. That would be quite neat. Make the transparency higher and lower to get that effect.
Looks really nice so far, though. 
Well fireflies do typically light up and then fade. You may want to use a Transparency number sequence for the particle system. You click the … by Transparency and you can make them flicker by setting the value up and down over time.