UED pyrotechnic engineers were firebats employed by the United Earth Directorate's Science Corps.
History
The zerg retreated to Tarsonis after their victory at Korhal over their former allies and the UED. The UED counterattacked with a pair of Slave Broods. The Slave Broods were controlled by scientists who were in turn protected by pyrotechnic engineers. The offensive collapsed when the zerg killed the scientists.
Game Unit
References
StarCraft: Brood War. Vivendi Games. Mission: Fury of the Swarm (in English). 1998.