Unique Alien Blaster.
Uses the rare Alien Power Cells as ammunition. In addition to its 100% critical chance and high damage, the Firelance sets its target on fire, causing 2 points of damage every 5 seconds. Pyromaniac and Xenotech Expert perks will affect this weapon.
The Firelance can fire 63 shots, about 6 cells, before burning out.
The Firelance can only be obtained in the "Unidentified Flying Debris" random encounter. See Fallout 3 random encounter locations (type A) for locations where this event may take place (via the wiki link at the end of this description). The encounter only occurs once per player character.
Things to note:
The Firelance can get stuck on buildings, like a water tower in the wasteland (use grenades to get it down).
If the Firelance Event took place and you are having trouble finding the weapon, you might want to use Dogmeat to search the area for the Firelance. And dont worry, Firelance wont disappear if you travel away to get Dogmeat. Only risk is that if the Firelance Event took place near hostile NPCs, they may pick up and use the Firelance. In that case, you can find it from that NPC's dead body.
Normally, this weapon is weaker (but still very hard-hitting) than the Alien Blaster due to its burning effect. However, with the pyromaniac perk, its power exceeds that of the Blaster. It still wears down faster, though.
If the event occurs while passing through a rocky part of the Wasteland, the Firelance may drop onto rocks and not be retrievable by Dogmeat, as he often gets stuck around rocky areas. (the same with ammo).
For detailed statistics and all other info:
FALLOUT WIKI: http://fallout.wikia.com/wiki/Firelance