Just create a bookmark and add the following as URL (make sure to change the playback rate ("2" for 2x speed) with whichever speed you prefer.
javascript:(function(x){document.querySelector('video').playbackRate=x})(2)
Select the browser bookmark while watching a video (YouTube is a good place to test) to speed it up.
Notes:
- Firefox seems to support up to 4x before it cuts out the audio.
- Limited also by bandwith and server/client-rendering speed.
- This does not work with most iframe configurations.
- Also note that the site UI (ex: speed selector on YouTube) will likely not update.