There are currently problems reported with the Firefox account service used to sign in to Sync. The Mozilla Engineers are looking into this issue and best is to try again later or one of the next days to see if the service has been restored. See Mozilla — Cloud Services Status: *https://status.services.mozilla.com/ <blockquote><pre>Firefox Accounts: We are currently experiencing problems with FxA logins. Engineers are looking into the problem.</pre></blockquote>