I installed this specifically for Spotify playback. For some reason, Spotify doesn't support speed control, even for podcasts, on their website. Once you install this addon, you will have to "reload" the Spotify site, then it works! The controls are simple and easy to use, and you can adjust the speed while something is playing. Haven't tried it with other sites yet, but the other site I use the most (YouTube) already supports playback speed control.