How do I centrally-manage multiple Fireboxes from a single user interface? 

With WatchGuard Cloud, you can monitor and configure multiple cloud-managed Fireboxes from a single user interface. Templates enable you to easily manage shared configuration settings for multiple devices. For more information, go to Manage Fireboxes and FireClusters in WatchGuard Cloud. 

Alternatively, for locally-managed Fireboxes, you can use the WSM Management Server to centrally manage multiple devices. For more information, go to Centralized Management with the WSM Management Server.