Tube Audio Options+ by 0x2F

Open source: https://github.com/schdie/we_firefox_ytop_mv3

On desktop it adds a toggle in the lower right corner of the video player to enable audio-only playback, allowing you to save battery by disabling the video rendering and bandwidth by downloading only the audio stream.

On mobile it adds a 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.

Works best alongside an adblocker like ublock Origin.

Limitations:
Doesn't work with live content.
On desktop using the YT miniplayer may have unexpected results.
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 (and also depending if you have autoplay enabled or not) it may render the video first but just for a couple of seconds.

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.