Firelance

Firelance is a lightweight, yet powerful weapon, that uses same ammo type as the Alien Blaster. It also sets target on fire, like Flamethrower or Shishkebab, giving it -2 HP effect for 5 seconds.

Appearances
Firelance appears only in Fallout 3. It can be picked up off a random Enclave officer.

i found it running east from the regulators base when i came close to a broken bridge over me i heard noises and thought it was the enclave but the alien ammo rained down on me along with this gun

Can also be found on top of one of the Meresti Trainyard buildings. Jump from garbage bins to get on top. (confirm this please)

Can also be instanced as below in the area to the top left of the map near the vault tech sign and the Landmark (name>?) in the area.

It can also be found if you have high enough luck (Confirmed). You will hear something akin to a jet flying over followed by something exploding overhead. You will also hear something crash and skitter across the ground, similar to knocking over a large object. At night the explosion looks similar to a plasma rifle shot impacting, during the day it looks like a frag mine going off.

After the explosion, the immediate area will be strewn with Alien Power Cell ammo and the Firelance. Unfortunately, it is in no particular pattern, and you need to find the ammo (there is 24 total) and the Firelance on your own. HINT: Dogmeat is incredibly useful for this situation, as he can be told to search for a weapon and he'll fetch you the Firelance. Dogmeat can also search for ammo and retrieve the alien power cells. Alternatively, wait until nighttime and look for the ammo, which glows blue in the dark.