This happens when your email provider automatically sends a error message back to the Firefox Account email system. Unfortunately, I've seen Yahoo frequently email back with an error message, even when the email goes through correctly.

In other cases, this can also happen if you have set an automated reply (sometimes called vacation reply on some email providers) because Firefox interprets this as an error message and assumes the email wasn't sent properly.

Once your account gets locked there is a waiting period (I want to say 24 hours) before it will be unlocked and you will be able to attempt to request the codes again.

If you continue to have issues with this, you may want to try changing the primary email address for your Firefox Account to another email provider.

Hope this helps.

Hello,

    I still CAN NOT sign in and sync up the Firefox.  I get the same account locked error after 24-hr.  
   Please help!

See also:

• https://support.mozilla.org/en-US/kb/forum-response-firefox-sync-verification-email-faq
• https://support.mozilla.org/en-US/kb/change-primary-email-address-firefox-accounts
• https://support.mozilla.org/en-US/kb/add-secondary-email-address-firefox-accounts