You could go to monitor.mozilla.org to check if what the email is saying is true. The same goes with any email wanting you to take action, if you don't believe it, why not go see for yourself.

According to https://support.mozilla.org/en-US/kb/mozilla-monitor-faq#w_how-do-i-know-these-emails-are-from-firefox-not-a-hacker, the email address that you provided matches up, BUT the sender email can be easily manipulated, so it isn't a reliable way of determining an email's legimitimacy. (email spoofing)

I received an email from breach-alerts@mozilla.com informing me that Mozilla Monitor⁩ discovered a data breach that includes my personal information: Unfortunately, I don't know if this is a legitimate email from Mozilla. Can anyone confirm if it is or is not?

breach-alerts@mozilla.com itself is legitimate.

You can also search for existing breeches at https://monitor.mozilla.org/breaches

https://support.mozilla.org/en-US/kb/resolve-breaches-firefox-monitor https://support.mozilla.org/en-US/questions/new/monitor

According to https://support.mozilla.org/en-US/kb/mozilla-monitor-faq#w_how-do-i-know-these-emails-are-from-firefox-not-a-hacker

That link should be "how do I know these emails are from mozilla not a hacker" since Firefox is a web browser.