click firefox addon s button in toolbar

can someone please help me how to click a firefox button added by add on? the dd on is soundcloud player url: https://addons.mozilla.org/en-US/firefox/addon/soundcloudplayer/?src=search
i d like to click next song button thanl you!!! see screen shot:
https://www.copy.com/browse/copy/Screenshot/Schermata%202014-06-29%20alle%2001.28.17.JPG