The Video Speed Controller extension for Firefox allows you to control the video playback speed of any HTML5 video in any webpage. It displays an overlay at the top-left corner of the video widget with buttons to increase or decrease playback speed. The extension also supports keyboard shortcuts, but I have not found any interest to use them.
I find most lectures and tech talks to be slow at 1x and tend to watch them at 1.2x or 1.5x. This extension has literally saved several hours of my life this year by allowing me to speed up playback on Youtube and several other sources.
Tried with: Firefox 71.0 and Windows 10 x64