This Crystallized Fire farming guide will show you where I farm Crystallized Fire in Wrath of the Lich King Classic. Eternal Fire comes from combining 10 Crystallized Fire, so if you need Eternal Fire, you have to farm Crystallized Fire.
The Storm Peaks
Farming Seething Revenants in Storm Peaks is a good source of Crystallized Fire.
You have complete the Sons of Hodir intro quests to unlock this farming place otherwise you will see a bunch of level 80 elites patrolling this area.
You can also farm Wailing Winds inside the marked cave. They are confined to a small cave so you can often pull 2-3 together and AoE them down. This place is best for DoT classes or Classes with good AoE.
Killing Raging Boilers in Borean Tundra is also a good way to farm. These mobs have a bit lower drop rate, but they are low levels mobs that you can kill quickly, and you will never run out of mobs to kill because they respawn quickly.