Greasy Fork is available in English.
A lightweight and non-intrusive userscript that control video playback and set the preferred player size and playback quality on YouTube.
< Feedback on Yays! (Yet Another Youtube Script)
For anyone else looking for a way make custom speeds, you can try Menu Wizard for Firefox.
That is how I got 1.05, 1.10, 1.15, 1.20, even 0.9 :smiley:
Custom speed
Any chance of adding custom playback speed.?