Fire Spring is a location in Mother 2. It is the last "Your Sanctuary" zone, and it is found in the Lost Underworld. It is also after Ness gains the melody that he is transported to Magicant, but only if he has all of the other melodies at the the time.
Fire Spring, as it's name implies, is a lava-filled dungeon with ropes and caves that lead to the top of the mountain. It's guardian is the Carbon Dog, who later becomes the Diamond Dog. This will always happen in the middle of the battle. The enemies inside are obviously fire-type, and are weak to PSI Freeze.
The tip of the mountain, where the "Your Sanctuary" is located, is a small volcano that has a river of lava pouring out from the front of it.
The vision Ness has here is he feels like he's being watched by himself as a baby.