I like this a lot, but lately it seems like pausing notifications or changing the notification schedule doesn't work. So it isn't really working as designed. Not sure why. Hope you can fix this when you can!

Useful little tool to get a break once in a while. Settings page is customizable, interval (in minutes), pop-up text and notification sound free to chose.

It doesn't save settings in between Firefox updates. Once you got a new version you gotta redo all your customization.
Also, if you have multiple screens and switch your Firefox window to another screen, the pop-up keeps showing up on the original screen & doesn't switch with your Firefox window. A little annoying :)