The Fire Heart was the last dragon egg, and the dragon that hatched from it.
At some point during or after the Great War between Good and Evil, the egg was sealed in a puzzle chest and hidden away in Mt. Fuego, alongside a monster to guard it. The scroll showing its location was hidden in the depths of the Cimmerian Forest, and the map showing the way to the scroll's location was lost to history, as was the truth of the chest's contents; all that was known at the time the series began was that the ancients believed that it was a source of tremendous power.
Finding the Map
The map was eventually located by archaeologists, and was planned to be examined for its meaning at a museum. Necrolai learned of it, and sent the Taxi Cab Monster to steal the scroll. It met Necrolai in the parking garage to hand it off, and the Rangers arrived shortly afterwards. Thanks to the garage's surveillance footage, they gained a copy of the map and Udonna deciphered it. The Rangers raced Necrolai through the forest, locating the scroll that showed the secret of the Fire Heart before Necrolai arrived. After a struggle, the scroll was torn in two, and Necrolai and the Rangers each escaped with half of the map.
Treasure in the Mountains
For a time, both parties try to decipher their halves of the scroll, to no avail. Chip finds the answer when he sees Xander staring in the mirror, reasoning that the two halves of the scroll were mirror images, and putting the scroll next to a mirror, they recognize Mt. Fuego. Leelee overhears Chip's epiphany, and passes the news on to her mother. The Rangers locate the Fire Heart, destroy its guardian, and fend off attacks from a Styxoid and its army of Hidiacs that Necrolai sends to retrieve the chest. However, a one-hour timer starts on the box when the rangers remove a scroll, which states that if they can't solve the puzzle before the timer runs out, the Fire Heart will be lost forever. The rangers exhaust nearly half of the time trying to solve it themselves, then Chip decides to pass the chest over to their boss, Toby, who they knew to be an expert at solving puzzles. Fortunately for them, Chip's faith was well-founded; with seconds to spare, Toby determines which button on the chest will unlock it. Vida presses the button with only one second to spare, and the chest opens. The Rangers thank Toby and return to Root Core, Xander distracting Toby from wanting to learn of the chest's contents by telling him a riddle. Udonna identifies the contents of the chest as the last dragon egg.
The Last Dragon
The Dragon Egg continued to grow for some time, until it finally reached the point where it was ready to hatch. While a Dragon book said to just crack the egg open with a hammer, Clare would not have that happen. Jenji then instructed her to sit on the egg and wait for it to hatch. Sure enough, the egg did hatch and this dragon appear, and it now calls Clare it's mama.
The Rangers soon became very interested in little Fire Heart. So much so, that Jenji became jealous of the dragon. The genie took Fire Heart and left him out in the forest to survive alone. After learning of this, the Rangers and Jenji went back to find Fire Heart, but he was nowhere to be found. What they didn't know was Phineas had found the baby dragon and taken him in and given him the name PJ (for Phineas Junior). The little Fire Heart then dubbed Phineas as his Daddy.
When Phineas awoke from a nap, he was then surprised to find Fire Heart had left him. It was then that both Clare and Phineas discovered they were looking for the same dragon, and Fire Heart then showed himself.
Now Fire Heart has grown to his full size, and is very powerful. So far, he has helped the Rangers by flying Solaris Knight to the city, when he was needed to save Jenji. He is also capable to transporting the Rangers to other dimensions, as he did when the Dark Wish was active.
In time, Fire Heart will prove to have a great deal more powers and abilities.
After months of care and training, Fire Heart's true destiny has finally come to pass. When he and the Red Ranger join together, they form the Red Dragon Fire Ranger.
In Operation Overdrive, one of his scales was used to power the Defender Vest.