Getting the firelance is triggered by a random event, going to certain places in the game triggers random events, such as going to the 2nd level of the temple of the union, i read on the internet somewhere, a guide to get the firelance, so i tried it
It said, go to the 2nd floor of the Temple of the Union, so i went, it said you should hear a loud bang, like a barrel being kicked accross the floor, and i did, i looked down onto the street, and there was something glowing, it was an Alien Power Cell, so the event had happend, though now i had to find it, i hadnt got dogmeat yet, and i didnt want to risk going for him, but in the end i risked it, i came back, the stupid mutt found a knife first, then the Firelance, it worked for me 1st time, in my 2nd game save, i wasnt even trying to get it, the random event happend at broadcast tower KB5, which is on top of a freaking steep mountain, luckily it landed right next to me, i had to get dogmeat to find the rest of the ammo though, which he did, he ran to some raiders first and got killed
Credit to Pity for Awsome Sig