Where does Apple Configurator store ipsw downloads?
Since Apple Configurator 2.0 they are downloaded in:

~/Library/Group Containers/K36BKF7T3D.group.com.apple.configurator/Library/Caches/Downloads/ACUDownloadFileOperation/


and stored in:

~/Library/Group Containers/K36BKF7T3D.group.com.apple.configurator/Library/Caches/Firmware