Okay, full disclosure: they are not real fireflies :)
I taped a big thick piece of yellow and green plastic to the led flash of my phone, then I had to dig around for apps that provided strobe lighting but with short duration (most that I tried were too long and it left a trail, not a single dot).
After finding such an app, I went to a deserted field and measured for the ambience. It's a bit over 3 minutes at f/5.6.
Then I just walked around (ducking mostly) and waving my phone around, to put lights everywhere.
Editing is just some color tweaks for a greener feel.
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