Version 62 of Firefox will integrate a new add-on called Firefox Monitor that basically informs users if their email accounts have been compromised. The add-on uses the popular Have I Been Pwned database backend to check if the account has been compromised in data breaches.
https://www.notebookcheck.net/Firefox-62-comes-with-Firefox-Monitor-that-notifies-if-you-ve-been-pwned.324598.0.html