This extension is brilliant.

Here is my use key:

When I open youtube video, stop it, or even didn't start, it makes firefox to register BuiltInSpeakerDevice as busy and prevent mac from deep sleep. So it drain your battery until zero!!!

To check it, try terminal command "pmset -g assertions".
If you see here something like

pid ... coreaudiod ... PreventUserIdleSystemSleep named: ...BuiltInSpeakerDevice ... preventuseridlesleep

and
pid ... coreaudiod ... PreventUserIdleSystemSleep named: ...AudioTap ... preventuseridlesleep

right after openning youtube tab (even if video stopped)

That's it. This extension unload all background tabs with no audio without your attention.
Love it.

By the way, FF remember scroll position of any tab, so when you open that old tab, it will reload quickly.

What if author add site list to reload more often, and other sites to reload with longer period.
I will configure youtube domain for 10 minutes and other sites for hour or two