It does exactly what it is supposed to do and it does it right. One little issue I have under firefox 74.0 on windows 10 is that I have to click on the icon twice for the panel to show. I hope it's just a temporary glitch.

Update 04/09/2020: The glitch seems to be resolved! But now I've noticed that sometimes the bell rings and sometimes it doesn't. Tends to happen when I've had my laptop/browser running for a long time (i.e. days between restarts) but plenty of sleep cycles.