I have made several fixes and improvements, based on those by Mitch Crane.
addons.mozilla.org/en-US/firefox/addon/sleep-timer-reload/

New features:
- apart from "Close" or "Mute", the new default option is "Reload". Useful if you are listening to music or radio, and you want to stop the music but be able to restart it later. Another option would be to "Unload" the tab, but I couldn't find the way to do it in the API, and "Reload" does pretty much the same.
- more precise and logic behavior
- better usability on smartphone; better tested ("Mute" does not work on Android)
- the duration may also be entered with the keyboard (including on Android)
- icons showing that the timer is counting down (stopwatch in the pop-up, red icon in the browser toolbar)
- remove (if possible) the "beforeunload" event, before Reload or Close.

This add-on does what it says. Sometimes.
Fortunately, Mitch Crane has offered a fix for this at https://addons.mozilla.org/en-US/firefox/addon/sleep-timer-fix/

I was searching for this kind of add-on to limit access to the web version of Yo*tube Kids for my kid. Since it has a timer on the app version only, while there's none if we access the web version.

Thanks to developer.