Kill Me Love Me - there appear to be fireflies in the opening theme song and one scene had what looked like a firefly but it was not the focus of the scene. Does it count? I believe it was episode 2 or 3.
They made it seem like it wasn't due to the additional drops but something else that was the real murder weapon or the real murderer. Not sure why they left it ambiguous like that.
https://watchasia.to/drama-detail/crime-scene-cleaner-2024
54:20 Several fireflies at the edge of the cliff