Mozilla has found out that some Firefox account passwords that are use for the Sync service are compromised and misused. The passwords of affected accounts have been reset to protect you and an email is send to notify you. See: *https://blog.mozilla.org/services/2016/04/09/stolen-passwords-used-to-break-into-firefox-accounts/ Always make sure never to use the same password for more than one website, but always use a different password. * https://support.mozilla.org/kb/create-secure-passwords-keep-your-identity-safe * https://en.wikipedia.org/wiki/Password_strength