https://support.mozilla.org/en-US/kb/firefox-monitor-faq https://support.mozilla.org/en-US/kb/firefox-monitor

Reply to James’ response re Firefox monitor: my query was not about whether Firefox monitors e-mails, but whether the e-mail I received from Mozilla.com about a data breach was genuinely from Mozilla, or whether it SWd a scam from someone impersonating Mozilla

Well Mozilla owns the *.mozilla.com, and *.mozilla.org domains so the *@mozilla.com email is real.

The above links explains why you got the email among other things.