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.

Note: This weapon is found as a rifle on this Wiki, even though it looks the same as the Alien Blaster.

Appearances
The Firelance appears only in Fallout 3. It is primarily obtained from a random event, though it can sometimes be found on random NPCs in the Capital Wasteland.

The random event has a better chance of happening with a higher luck skill. You will hear something akin to a jet flying overhead, followed by an explosion. 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. Note: I found it in front of Super Duper Mart being used by a raider to fight a Enclave Eyebot. I did not see or hear any jets overhead, nor an explosion.

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.