Open source: https://github.com/schdie/we_firefox_ytop_mv3 This extension adds the following functionality to youtube.com and m.youtube.com:

Works on Desktop and Mobile.
Works with the regular video player and the mini-player.
Save battery and bandwidth.
On desktop use the toggle in the lower right corner of the video player to enable audio-only playback.
On mobile use the semi-transparent floating button on the lower left part of the screen to enable audio-only playback, you can also "leave" the page and playback should continue normally.
Seamlessly switch between audio-only and video playback using the toggle on desktop or button on mobile.

Limitations:

Doesn't work with live content.
If autoplay is not enabled there may be some strange behavior.
Depending on your network connection on some rare cases it may take a couple of seconds to get the audio-only stream, until it does it may render the video first but just for a brief moment.

Works best alongside an adblocker like ublock Origin. Permissions: "Access your data for sites in the *://youtube.com domain" is required to change the video/audio source of the player. "Access your data for sites in the *://googlevideo.com domain" is required to capture the audio-only stream.

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