Getting the Firelance relies on a random event where a UFO explodes overhead, scattering the blaster and some Alien Power Cells over the area. It helps to have Dogmeat on hand to find the cells. It is recommended to find the Firelance during the day and then get the APCs at night, as they produce a green glow. The Firelance is an Alien Blaster modified to do fire damage, so it benefits from Better Criticals and Pyromaniac.
The random encounter's chance of happening depends upon your Luck stat.