Data breech on an account I never set up

If you never created an account with Post Millennial, I wouldn’t assume that someone is using your email just because you received the breach notice. Your email address could have been included in a leaked database without you ever having an account there. I’d start by checking whether the notice is actually from Mozilla and not a fake email. Don’t click any links in the message. You can also check your email account’s recent login activity and change your password if anything looks unfamiliar. If you use the same password on other sites, change those too. If you’re not seeing any suspicious activity on your email account, the breach notice by itself doesn’t necessarily mean someone has access to your email.