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.