The new geckodriver v0.17.0 has a new way to install addons as mentioned here:
POST /session/{session id}/window/fullscreen to invoke the window manager-specific full screen operation
POST /session/{session id}/moz/addon/install to install an extension [Gecko only]
POST /session/{session id}/moz/addon/uninstall to uninstall an extension [Gecko only]
How can I use these endpoints to install my addon to firefox for my selenium tests?