here's an article about the add-on:
https://www.ghacks.net/2018/08/25/firefox-62-firefox-monitor...
They are missing the about:config setting.. but it's early. Might be a bug, or maybe its not being used(?)
Eventually it'll display a popup in the UI that will notify you if the website you're on has been breached. I think we can all agree that would be useful.
Edit: Here's the code for the add-on: https://github.com/mozilla/blurts-addon