Fire Lake is a lake presumably notable for its fiery appearance when reflecting nearby patches of fire. It was briefly featured in Secrets of the Masters when Oogway was escorting Rhino, Ox, and Croc to Wu Fortress to defeat the Wu Sisters.
Fire Lake is a large landmark somewhere in China. Since it was only briefly mentioned in Secrets of the Masters, there is hardly anything known about it. Although given as how there is fire everywhere the seems to surround the lake, it is possible that this lake could really be a giant lava pit.
There is nothing else known about Fire Lake other than the events shown in Secrets of the Masters. After encouraging Rhino, Ox, and Croc to work together in stopping the Wu Sisters, Oogway escorted the trio to Hubei Volcano where the sisters' fortress was located. They passed by this lake along the way.