Hi Angelica,
Tripod, yes! I arrived at the exposures purely by trial and error. There is this three way balancing act of firefly light, ambient light, and enough time to for an adequate number of fireflies to light-up.
I found that closing down a bit to 5.6 or so at ISO 1100 for about 3 seconds worked well about 30 minutes after sunset. So, it was pretty dark.
Thanks for the comments about this and the other images. I didn't spend a terrible long time in post processing on any of these images. With the high ISO and long exposures, there is some especially nasty grain that could be much improved with some time consuming tweaking I think.
I have one more version from last night that hopefully I'll get around to posting today!
Take care.