Essence of Fire Farming Locations (Classic - WotLK)
Essences of Fire are one of the cheaper Essences on the Auction House despite being used in many different high end recipes. The reason for this is they're extremely common in the most popular raid of Classic WoW, Molten Core. If you actually need this item, I would recommend you buy it from the Auction House rather than farm it.
If you choose to farm this item, keep an eye out for Elemental Fires when you do. They'll often be worth more than the Essence of Fires (at least until phase 5) since they're used in creating Greater Fire Protection Potions. Once Phase 5 releases you'll be able to purchase the Transmute: Elemental Fire recipe with Thorium Brotherhood reputation. That will allow you to transmute Heart of Fire into Elemental Fire which is when the Elemental Fire market bubble will implode.
Felwood - Essence of Fire Farming Location
Felwood is one of the two places you can farm Essence of Fire. You'll find green fire elements scattered around the craters at the area just north of where it says Bloodvenom Falls on your map. Mixed in with these fire elementals you'll find earth elementals too. They are elite and in their lower 50s, a little bit tough to defeat but if you do they have a chance of dropping Essence of Earth. Head over to my Felwood Essence of Earth Farming Guide for more information about that.
Un'goro Crater - Essence of Fire Farming Location
Un'goro Crater is technically the best location to farm Essence of Fire once the Elemental Invasions start. The fire elementals you want to hunt are surrounding the volcano and on the path leading up it/along the top. During the elemental invasions you'll find even more fire elementals surrounding the volcano in the center of Un'goro.
Note: On the WoW Classic re-release the Elemental Invasions began between phases 2 and 3 on December 10th 2019.
There are enough fire elementals here to support multiple people farming at this location, during an elemental invasion you'll find twice as many fire elementals too. A word of warning, I would recommend you avoid the cave at the tippy top of Un'goro Crater as it has an elite fire elemental in it by the name of Blazerunner. He hits about as hard as the Fire Destroyers in the second half of BRD and will whoop your butt if you're unprepared or without a group.