When I accessed that website with Firefox on GNOME 3, I got the following information:
To control GNOME Shell extensions using this site you must install GNOME Shell integration that consists of two parts: browser extension and native host messaging application.
So I've added
/etc/nixos/configuration.nix, rebuilt, rebooted and installed the "GNOME Shell integration" Firefox extension offered by https://extensions.gnome.org. With that, https://extensions.gnome.org displays the error message
Although GNOME Shell integration extension is running, native host connector is not detected. Refer documentation for instructions about installing connector.
What am I missing?