@zkhcohen: I tried with Opera and Chrome, and yes, then I can login. It says 'Ok. This window can be closed now.'
Yet, in the plugin panel, it says: 'No tracks found'.
CORRECTION: when I play a song of a local playlist, it shows the song and whether it's part of an album in Spotify, if I follow the artist and so on... (and I can change that... add... follow...)
Which is great!
Yet, I would like to be able to play Spotify playlists: is that possible?
(I don't see that for now, but I might be overlooking something.)