This is the position in which mating pairs may be seen most often. But, how do they get into this position? Surely, males and females can't be crawling backwards until they bump into each other. Remember that prospective firefly mates locate each other using light signals. Males fly around flashing their lights at species-specific intervals and the females waiting on the ground respond with their own species-specific flashes.
Here is the 1st video I ever made of a pair of fireflies locating each other and then initiating copulation.
The firefly seen in the beginning is the female. She is flashing to a male that was initially not visible. He soon appears on the left, flashes once in response and then flies to the leaf the female is on and climbs on her back. The pair spends about 45 seconds in that position. During that period the male can be seen caressing, if I may anthropomorphise a bit, the back of the head of the female. Its antennas are also quite active, while the female, as far as we can tell at this magnification, is quite immobile, although I suppose she is monitoring the male's activity patterns. Are they making one last check to make sure that they are of the same species? Somewhere along the line, the male enters the female and then turns around to assume the typical mating position.
These are probably a species of Photinus, but that's as far as I can go with an identification.