Method 1 - Megaflip Edit
The fastest and most commonly used method. At the top of Death Mountain Crater, above the climbable wall, simply Megaflip down to the Ocarina Warp platform and go on to enter the Fire Temple.
Method 2 - Hover Boots Edit
From the enterance from Goron City, go to the the left stake of the wooden bridge near the edge. Equip Hover Boots and aim for the corner of the small mountain. Roll off the edge to get on top of the mountain. From there you can just walk over to the Ocarina Warp and into the Fire Temple. You can collect the Heart Piece up there if needed.