Ooof, I love it, so atmospheric, great ambient music, and brutally hard; I can't beat it. Made it to four fireflies. I quickly started not pushing the fireflies at all, since they will (permanently?) move slower afterwards. Maybe a life lesson.
Perhaps a pull on the fireflies would be nicer as a game mechanic? It's tricky to predict their trajectory and lure the monsters into it without getting far into the darkness, where new monsters might be. The hitbox also feels unfair at times, with the stakes of getting hit being as high as they are it would be nice to feel every hit is justified.
Having a new firefly at exact minute marks would give the player something concrete and known to look forward to, now I sometimes feel a bit frustrated and desperate when I'm running around on one heart with no potions dropping despite having multiple upgrades on those.
Somehow I felt that the difficulty first seems to get easier and then harder again, I wonder if more people feel this way.
Your game is actually kind of similar to mine, and you might also enjoy this one with a similar mechanic: https://ldjam.com/events/ludum-dare/54/vade-ratro