Sounds like Firefox just updated after you grabbed your installer, or you used an older version's installer. Whichever the case, the Firefox account is voluntary to use and not required for updates. You can update using these instructions: https://support.mozilla.org/en-US/kb/update-firefox-latest-version. If you want to access your Firefox account, use this link: https://accounts.firefox.com/. I can't remember how the confirmation and activation email goes, but I think it just links to the login page - once there, you log in and you should be good to go!