Resetting the sync password, or changing its e-mail or password, 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.

Does the system/profile you want to sync with have the passwords?

In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.

FredMcD said

Does the system/profile you want to sync with have the passwords?

yes I do have

Open the Password Manager and find what you want.

Resetting the sync password, or changing its e-mail or password, 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.

I'm sorry, but how is losing all my bookmarks because I forgot a password that I use once every blue moon a "security feature"? What on Earth do people bookmark that needs this level of paranoia?

That is up to the users.

Losing data on the Sync server when you change the password of the Sync account is not a security feature, but is how Sync has been designed. Your personal data is encrypt locally with a key derived from the password before uploading and only this key can decrypt your data on the Sync server. A different password means that a different sync key is generated that makes it impossible to decrypt your personal data. If you reset the password that you should see a warning.

I would like to change my password, but, do not remember old password.

Hi backstrm

If you do not remember the password for the Firefox Account then you will have to reset the password.
If you use 2FA then your only option is to create a new account with a different email address.

• https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information
• https://support.mozilla.org/en-US/kb/what-if-im-locked-out-two-step-authentication