It wold appear you must use the Login link at the top-right of https://thimble.mozilla.org/ https://thimble.mozilla.org/. That sends you to a login page with a

It wold appear you must use the Login link at the top-right of https://thimble.mozilla.org/ https://thimble.mozilla.org/. That sends you to a login page with a bunch of extra parameters after the https://id.webmaker.org/login? https://id.webmaker.org/login?. e.g. https://id.webmaker.org/login?client_id=wm_id_wMvw9BSwq2A480dxr&response_type=code&state=e9baaf0a-07e0-497d-85d7-72a7eefc36&scopes=user%20email https://id.webmaker.org/login?client_id=wm_id_wMvw9BSwq2A480... If you try to login just from https://id.webmaker.org/login https://id.webmaker.org/login you will get the error you saw.