Just found this Firefox add-on via DuckDuckGo. Exactly what I was looking for! Works great from what I've seen so far. I like that I can change the playback speed directly on the screen just by hovering the mouse in the playback window and clicking the control it adds (note that the control appears/disappears in sync with YouTube's playback control panel, except in full screen mode...). Having the ability to change speeds up/down using the keyboard is nice too (if I remember it works that way!). The control also allows you to specify the increment at which the speed is modified (it appears this came as a result of reviewer requests) and shows me that the developer cares about the functionality of their add-on. Best of all, it's free. Kudos to Toxblh, I'll spread the word!

Super! YT playback rate shifted at the touch of the shift key and the mouse wheel!

it is great

Works very well. Simple and to the point.
Perfect solution after the Enhancer for YouTube™ got pulled from Firefox by the author.

This extension is really well made and works as promised. Love it.

Simple and effective. Allows you to increase the speed beyond the max x2 allowed by YouTube. Works really well, thank you to the add-on creator for making this.

Minimal & convenient

Funciona bien, exactamente igual que en Chrome. Siempre pongo ocultar siempre el panel y uso las teclas + y - del teclado para regular la velocidad. No entiendo por qué tiene tantas notas negativas.

Лучшие! Спасибо огромное за Ваше старания.

Works well and is easy to set up.