This extension is used to set a custom playback speed for YouTube videos outside of the default options given by YouTube. It has finer granularity and some other nice features. It is available for Firefox and Chrome. If you want to know more, please visit the respective extension (or add-on) pages from
The Chrome Web Store
This extension was started with the help of Extensionizr. As well as the documentation provided by the Google for Chrome Developers and the documentation from Mozilla