YouTube Section Looper (opensource) автор Sudhir Babu Nakka

YouTube Section Looper is an opensource project that adds a small control panel directly on YouTube watch pages. Set the start and end timestamps (by button or manual entry), choose how many times to repeat, and play the loop automatically. Settings are saved per video so you can return later and continue looping.

https://github.com/sudhir-asuracore/firefox-youtube-looper

(content looper, video looper)