I know I am going to sound stupid, but I failed to find where the settings for this extension are. Neither the toolbar icon, nor the sidebar, nor the Firefox's extensions management page seem exposing any.
Main menu is the top left button inside the extension. In that menu you can enter Settings. Then in settings you can click the category to see a list of other categories, or use the arrows.
I see. Thanks. It just says "Tabs" so I couldn't guess it's there intuitively. Perhaps you might want to add a downward arrow to the button right side to hint it's a drop-down menu.