I really wish it actually looked like the screenshot. It does not show the currently playing track, artist, album name or artwork. Nor does it show the length of the track or time elapsed/remaining.
That being said, the scrubber does work, but you're really just blindly picking a point in time.
The volume slider kinda works, I guess? You can click and drag it to adjust volume, but it seems to have weird invisible boundaries that snap it to either max or something quiet. It doesn't visibly move the slider, so you're blind for that too.
With all that in mind this is still a great extension since there's no other addon (that I know of; feel free to give me a reccomendation) that gives you playback controls so simply

Man, I tried to copy-paste text from you player, but selection was disappear immediately! Only if I press "ctrl+c" very fast - sometimes I can catch the text. Can you fix copy paste ability for you app? Very stupid spotify's guys made copy-paste "name artist or tracks" unavailable. And I install your app only for this HUMANE feature.