It's possible that was a phishing email designed to get you excited enough to click a malicious link or open a malicious attachment. So it's good that you are investigating it first. What did the email say it was, exactly?

However it is possible like you may have subscribed to one of Mozilla's premium products that you have not named. * https://www.mozilla.org/products/vpn/ * [https://support.mozilla.org/en-US/questions/new/firefox-private-network-vpn/form Mozilla VPN] * https://relay.firefox.com/ * [https://support.mozilla.org/en-US/questions/new/relay/form Firefox Relay] * https://developer.mozilla.org/plus * [https://support.mozilla.org/en-US/questions/new/mdn-plus/form MDN Plus] * https://hubs.mozilla.com/ * [https://support.mozilla.org/en-US/questions/new/hubs/form Hubs]