In this work, we develop a user interface to manage a swarm of large fixed-wing Uncrewed Aerial Vehicles (UAVs) for firefighting applications through a user-centered design process. We conduct Wizard of Oz studies with nine firefighters and drone operators to collect end-user data on their ideal system for use in a wildfire scenario. This data is then translated into features and design drivers implemented in a prototype interface for swarm firefighting. The interface is evaluated through a usability study in which six participants provide feedback and score the interface, resulting in an overall usability rating of ’Good’ with a System Usability Scale (SUS) score of 76.25. We find that end users in this application are more concerned with producing the desired effect than with controlling individual UAVs. This finding provides a useful insight for the design of future multi-UAV systems, suggesting a reduced emphasis on micro-managing the fleet.