"We need a fire."
Translation:Wir brauchen ein Feuer.
I think in this sentence "fire" is supposed to mean something like a campfire, so it's countable. A group trekking through the wilderness might light several fires: one in front of each tent.
I think the difference is made between fire as a "material" (hot stuff, plasma, whatever you like to call it) vs. fire as a specific mass of burning things (a campfire, a house on fire).
If someone needs fire as a "material" e.g. to light a cigarette, you'll use "fire" in an uncountable way: "He needs fire." / "Er braucht Feuer." ("Do you have a light?" = "Haben Sie Feuer?") In the same way you'd say, "He needs wood" / "Er braucht Holz" (as a material).
Similarly: "The first town house was destroyed by fire in 1567" / "Das erste Rathaus wurde 1567 durch Feuer zerstört", "Fire can be extinguished with water" / "Feuer kann mit Wasser gelöscht werden"
If you're talking about a specific fire ("a heap of burning things"), you'll say, "The fire destroyed the house" / "Das Feuer zerstörte das Haus", "Someone has to watch over the (camp)fire" / "Jemand muss das Feuer bewachen", "The fire went out" / "Das Feuer ist ausgegangen", ...
Note that the event of a fire is "der Brand" in German: "In 1567, the town hall was destroyed in a fire." = "Das Rathaus wurde 1567 durch einen / bei einem Brand zerstört."