The backend requirements (copr dnf repo and ineptly named chrome-gnome-shell) could be made more clear on extensions.gnome.org. Works fine once you waste 20 minutes trying to figure out all of the manual dependencies. Grumble grumble.
As said by mostly everybody, this addon requires a backend service, it does not work out of the box by just adding it to Firefox, and this is by design, so, for everyone which did not it yet, repeat with me:
Read the addon instructions! Read the addon instructions! Read the addon instructions!