I would suggest changing the password.

Resetting the sync password, or changing its e-mail, will remove all data from the servers.

This is a security feature. https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information

• I've lost my Firefox Sync account information - What to do
• I’ve forgotten, or don't have access to, my Firefox Account email address

Shut down the old sync. Then create a new account.

Note: You should change any login data that might be on the sync.

How can i lock hacked account?

Follow my instructions above.

The current Sync version doesn't use a recovery key (uses a key that is derived from the password) unless you mean recovery codes used by two-factor authentication.

Can't you no longer access the account at all?

Was the Firefox account actually hacked or did you get a message from Firefox Monitor that your email address was compromised?

• https://support.mozilla.org/en-US/kb/firefox-monitor
• https://support.mozilla.org/en-US/kb/firefox-monitor-faq

Note that you should use strong passwords that consist of random characters and never use the same password on more then one website.

• https://support.mozilla.org/en-US/kb/create-secure-passwords-keep-your-identity-safe