Firefox Monitor is a Mozilla service that has partnered with Have I been Pwned to alert users when their email address has been discovered in a data breach. Starting in the current Firefox Nightly build for version 67, Firefox Monitor will now be integrated as a system extension. To test this feature, you can visit one of the known breached sites to see a notification. You can also add a new setting called extensions.fxmonitor.enabled to set it to true by default in about:config.
Source: https://www.bleepingcomputer.com/news/software/mozilla-firefox-67-to-warn-about-breached-sites-using-new-add-on/