“In order to create Firefox Monitor, we have partnered with HaveIBeenPwned.com (HIBP). HIBP is a valuable service, operated by Troy Hunt, one of the most renowned and respected security experts and bloggers in the world. Troy is best known for the HIBP service, which includes a database of email addresses that are known to have been compromised in data breaches. Through our partnership, Firefox is able to check your email address against the HIBP database in a private-by-design way. You can find Troy’s blog post on the partnership here”.

Source : Testing Firefox Monitor, a New Security Tool – Future Releases