“Joystick” refers to compatible USB input devices, which can be used to control Second Life in exciting ways for exploration, moviemaking, and more. Examples are the 3Dconnexion SpaceNavigator, other joysticks, gamepads, and so on.
Assuming you have a 3D movement device installed and configured, Joystick Flycam mode is enabled by pressing Alft-Shift-F. This moves your camera, rather than your avatar.
This window is fairly technical, although the default settings generally work well with no or minimal changes. For details, see this SL Wiki page.
One user, Xente Aeon, makes the following recommendations for the SpaceNavigator (in this JIRA):
Refer to this page in the SL wiki for other info, including known issues.
linux users do not need a 3dconnexion driver in order for the device to work in Firestorm. In fact, if you have a driver installed, it will conflict with the built-in support for the 3dconnexion device. So if you do not need the 3dconnexion device for anything else, uninstall the driver.
Alternately,you can stop it before running Firestorm. If you are using the free spacenavd driver, you can stop it by giving the following command in terminal:
sudo /etc/init.d/spacenavd stop