Plex Pace is a Firefox extension for controlling the playback speed of the Plex Web App player. Control the speed of your movies, tv shows, or lectures with ease. Includes preset speeds for slowing down or speeding up videos. Additionally, it allows you to set custom speeds and use key shortcuts.
https://addons.mozilla.org/en-US/firefox/addon/plex-pace/
MDN provides a wonderful article for running add-ons temporarily for testing purposes. https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Temporary_Installation_in_Firefox
Simply clone this project and follow the instructions provided to get up and running.
Feel free to contribute to this project.