hi,
1.- The extension just open the spotify page into the Firefox-sidebar. The sidebar push the content cause is design like a split view and its function is limited by Firefox, not me.
2.- The sidebar can't be minimized (just close) and can't keep its function (play music) on the background. I didn't see any extension doing that cause AFAIK it is impossible.
3.- That seems like a problem just for you, my extension don't affect the functionality, just affects a little of the UI of the web when the sidebar is smaller width than 800px.