This paper presents a design and implementation of an unmanned aerial vehicle (UAV) for outdoor firefighting application. The proposed UAV firefighting system consists of a self-designed quadcopter as platform, a transmission system to collect and release water, a real time kinematic (RTK) based navigation system and a mission control system to monitor and coordinate the UAV. In our proposed autonomous cooperative framework, the UAV finds an optimal path (with respect to distance and power consumption) to the fire spot at first. After arriving at the target on fire, the mission control system will guide the UAV to suppress the fire. The proposed framework has been demonstrated in the 2015 International Micro Aerial Vehicle Outdoor Competition, held in Aachen, Germany and help our team V-Lion obtain the second place. Our designed framework has successfully achieved all the requirements in firefighting mission even in a challenging wind gusts environment. The video of the firefighting mission can be found at: https://youtu.be/YREngbun7zQ.