The Professor's Protege - Myrlwood Firefly (Meldacio Hunter HQ)
First Available: Chapter 7
Other Requirements: Complete The Professor's Protege - Wyvern
Location: Meldacio Hunter HQ
Rewards: 10,000 EXP & Golden Hourglass
You can accept this quest from Sania once you've completed The Professor's Protege - Wyvern. Sania can be found at Meldacio Hunter HQ this time around, she's at the far end of the village closest to the tunnel on the northwestern side.
The Myrlwood Fireflies that we need are found in the back of Myrlwood, in the large area where you fight the Treant boss fight before the Royal Tomb. If it isn't night time then you'll want to use the nearby campsite to wait until nightfall, because you can only find/capture these Fireflies at night.
A screenshot of what you're looking for is found below, I am fairly certain that the next Firefly you need to collect only spawns after you've collected the one before it.