I made this Essence of Fire farming guide to help out players who want to farm the essences instead of buying them from the Auction House.
Essence of Fire
Molten Core is the best place to farm Essence of Fire, one full clear gives you around 15-20. Both the trash mobs and the bosses can drop essences, so kill everything inside the instance.
How to get to Molten Core?
Molten Core is located inside Blackrock Mountain which is a zone in Eastern Kingdoms between the Burning Steppes and the Searing Gorge, linking the two regions. You can enter from both zones, you'll have a partially open doorway, enter and follow the tunnel, when you come out you'll be in a large central chamber with lava in the bottom and a central rock chained to the sides.
Fly down to the location marked with X on the map. To enter the instance, you have to talk to Lothos Riftwaker, he will teleport you to Molten Core.