I usually install extensions for the GNOME Shell by visiting GNOME Shell Extensions in firefox, then clicking the ON/OFF button on the page for the extension I'd like to install. In Ubuntu, this works. In my fresh Debian 9 installation, however, clicking the button seems to have no effect. It switches to ON and I am asked for confirmation, though. But when I reload the page, the switch shows OFF. This does not work in Chromium, either.