Already found firelance?

You will know the Firelance Random Encounter when you hear an explosion and see a burning strike in the sky, like a meteor or a plane on fire, a smoke trail, the Firelance will drop near you, however, if you wait till night, it will have a better chance of showing the location since the ammo glows. you can do quests afterwards and come back to the location where you first seen the smoke and the firelance will still be there. I confirmed on my game file yesterday, I gave up and then found it in a gully nearby, I was on a cliff when the event happened. also at random times I heard explosions and assumed it was the event, however only when you see the smoke trail it is the event.

It does not sound like you had the firelance random encounter, but if you don't have Dogmeat yet, go get him from the scrapyard and take him to that location. I found most of the ammo, but he brought me the firelance and a few APC's I missed.

This happened to me near the Flooded Metro. Dogmeat and tell him to look for weapons. Repeat as needed, and he should bring you the Firelance eventually. Start out as close to the event as possible.

If you have dogmeat, bring him to the area and go to the "find weapons" selection when ur with dogmeat, and with a bit of luck it should turn up, btw at night the firelance glows orange (for the 360 version anyway, but idk if its different for the other versions) anyway, good luck dude

Okay First Off Firelance is found in a mini encounter (which are at specific locations all across the Capital Wasteland) but can only be encountered once.
You want to get yourself Dogmeat or finding this is gonna be hard.
Now Go to Tenpenny Tower and look for a ruined building a mini encounter should happen here.
Save far back from it and go towards it.
The Firelance encounter is an invisible alien pod interacting with the atmosphere and blowing up right above the building.
Now Talk to dogmeat and get him to find a weapon he will eventually bring you "Firelance".
If you dont have Dogmeat run around with a follower and he will pick up alien power cells aswell as the Firelance who you can trade with and get from him/her.
If you have neither then wait till night then get the mini encounter and the power cells should be visible as well as the Firelance if you use your pip-boy light.
NOTE: Doing this in the morning will be harder to find Firelance will be hard because it will be too bright to see the gun.

Mack is wrong about the specific locations and going to a building near tenpenny tower because the event is a random encounter but the rest is about right.