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. In the past, Firefox Monitor was a standalone service, but starting in Firefox 67 it will now be included as an extension. As part of a test in November 2018, Mozilla started displaying notifications to Firefox users when they visited a site that historically has had a data breach. You can see an example of one of these breach notifications below.
Source: Bleeping Computer