Faster Video for Firefox for Desktop that allows one to manipulate the playback rate of HTML5 videos. While right-clicking on a video allows one to change the playback rate to either 1.5X or 2.0X, this addon gives more control.
If a HTML5 video is playing, change the playback rate from the context menu by selecting video speed. You can access the context menu by right-clicking anywhere on the screen except the video itself. The menu will only be present if a single video is playing.
For Firefox for Android, see my addon
Faster Video Mobile
