For the mission Lighting a Fire in Anthem you will have to find 3 pieces of Ember. You get Ember from Harvesting Materials in the open world, it’s a bonus drop that can come from different harvested resources, such as Organic Materials or Minerals.
It may be random which harvest-object exactly drops the Ember. Below are 4 places where I found Ember. Again, these are just potential places for it and there may be some randomness to the Ember drops. Also keep in mind that other players may already have looted these objects before you. Just fly around the open world and loot all resource objects.
The following 3 locations are really close together, in the “East Gate” section where lots of Minerals are found:
If you don’t get Ember from these specific ones just keep exploring the map and harvest all objects for a chance to receive Ember in the progress. After you have 3 ember, go to the map screen and click on “End Freeplay” to return to Fort Tarsis. Give the quest giver Prospero the Ember to relight the Forge.