The Asmodian Entrance to Fire Temple in Morheim
The Three Keys is a level 32 Asmodian Campaign Quest, but it is gained and can be completed at level 27. Once this quest is complete, you will have access to the Fire Temple instance.
The Elyos Entrance to Fire Temple in Eltnen
The Fire Temple Key is a level 34 quest which can be acquired by finding a Fire Temple Key on one of the creatures in or around the Kyola Temple in Eltnen. Once found, right-clicking the key will give the quest, which can be completed by level 30 or above characters simply by talking with Actaeon outside of the temple.