I am using the most current release of Firestorm Viewer for Mac on a Mac Pro running OSX 10.11.6 and recently installed a 3Dconnexion Space Navigator.
When I plug in the Space Navigator, I can go into Firestorm preferences, enable Joystick, and it works beautifully - all without loading any drivers from 3Dconnexion.
However... in order to use the device in my Architectural CAD software for my "real life" work, I need to load the 3Dconnexion driver (10.4.4, currently the latest & greatest). When this driver is loaded, the device works beautifully with my CAD software but Firestorm