I use the same extension in Chrome as well. It supports Youtube Videos.

For Spotify Web, you need to install this add-on as well: https://addons.mozilla.org/en-US/firefox/addon/spotify-csp-clear/ The developer is working on improving this GlobalSpeed plugin so that it won't need Spotify CSP Clear in the future.

I'm using this plugin to help practice the bass guitar during covid quarantine - slowing down the songs/videos to help me to learn the song.