Before moving into this room use your lens to see all the pits spread all over the place.  Kill the bats and kill the laser shooting thing in the middle of the room by throwing a bomb at it.  The chest in this room contains a replacement Deku shield if you lost yours.  Go through the door and you'll be in the cage with the Shield-eater.  Avoid it when it tries to suck you into it and hit it with your boomerang or sword when it stops.  Kill the third and final gold skulltula in this room.  Now use your Ocarina and play the Prelude of Light to warp out of this hellhole and end up back in the Temple of Time.  Go into the Master Sword room and replace the Master Sword to warp to the future and continue your main quest.  Pat yourself on the back for getting 3 well-earned skulltulas and the Lens of Truth.

Goron City Revisited:

One way or the other, make sure you're now adult Link and head for Kakariko Village.  Go up to the Death Mountain Trail and travel to Goron City.  All the Goron's are gone except one rolling around on the third level.  Use a bomb to stop him from rolling and talk to him.  He'll give you the Goron's Tunic, which is resistant to fire.  Equip it.  Now go to the bottom floor and enter the King's room.  Pull the statue at the back of his room out of the way and enter Death Mountain Crater.  Use your hookshot to get across the broken bridge and on your way across it Sheik will come and teach you the Bolero of Fire.  Now continue on down and enter the Fire Temple.

Fire Temple:

Go up the stairs, hang a left, and enter the door.  Short story sequence here.  Now start hopping stones and make your way to the left of this chamber.  There's a cage there with a Goron in it.  Step on the switch to release him and nab the small key in the treasure box behind him.  Now hop on back to the door you came in and exit this chamber.  Walk straight across to the door on the other side and enter it.  Now in this large chamber make use of the bridge and platforms to make your way to the left side of the chamber.  There is a small hallway and a door there.  Don't panic if you fall in the lava, your new tunic keeps you from getting hurt at all for a few seconds to give you time to get back up.  Anyway, go through this hall and door, and down the next hall to another Goron. Free him and get the key from his treasure box.  Now go back to the big chamber and hop your way across to the other side (the right side).  Blow open the wall to find another Goron and key.  Now finally make your way to the other end of the chamber from where you came in to where there's a locked door.  Go through and you're at the Fire Elevator.