First, I’ll immediately link an album where I posted 14 locations I’ve found fireflies.
Now, here’s my discoveries about fireflies:
They do not respawn. Some articles I read mentioned the possibility of them respawning After two weekly resets, I’m confident in saying that isn’t the case.
What this means is that there is a set number of these collectibles. This could imply a grand unlock of sorts, but right now, this is all I can say.
Finding them is easier among ruins and/or near woods. It can be modern (for that time) or the Roman Ruins. Both seem to attract them.
Each spawn location spawns between 1-4. Best I can tell, the max will always spawn, but if you only gather two at a spot offering 4 before the others disappear, you CAN come back later to retrieve the two you missed.
Don’t bother with the horse when chasing fireflies. The fireflies are fast, but a lot of times, they tend to stop at the end of a path, like the tattoo designs. The animations of getting off the horse just take too long to be beneficial.
It’s hard to tell when it’s nighttime, as the lighting isn’t entirely realistic. I generally would look east and if the sky was looking kinda blue, I’d wait in place and meditate.
Finally, always travel at night and constantly pulse Odin Sight.
Those 14 locations are the best I could do on my own. I didn’t see any in Norway, and haven’t checked Vinland.