This is what I did to find the firefly - hope this helps!
1) Go to the location highlighted in the video north east of Grantbridgescire in England (there is a fast travel close by)
2) Once there make sure it is night - you can meditate to change the time
3) Find and catch firefly (press Y to catch)
4) Return back to camp and head to the waterfall in the north of the camp. As you face the waterfall on the shore look to the left by the building for the spot to release the firefly (by holding Y). It is just to the left of the spot to take a drink/wash