It's been working well overall. I actually wanted to report what seems to be a bug, but don't know how to do that besides rating the addon.
Issue: The timer appears to start back up after midnight, when it is supposed to be "off". I noticed this because I like to fall asleep with my laptop playing some videos, and set an auto-shutdown for ~30 minutes after I get in bed (a little before midnight). I disabled the timer before doing this, and found that the time was logged for the current day from the midnight roll-over.
I'm using Firefox 60.0.1 (64-bit)