Greasy Fork is available in English.
Add speed buttons to any HTML5 <video> element. Comes with a loader for YouTube and Vimeo
< Video Speed Buttonsについてのフィードバック
Work fine for me...
Tampermonkey + Waterfox Classic
Work Too with:
Greasemonkey v. 3.17 + Waterfox Classic
Doesnt work on YouTube.com