Resetting the sync password, or changing its e-mail will remove all data from the servers. The username and password are used to create the encryption. 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 '''Note:''' Only the data on the servers is cleared. The profile data is not changed or removed.