If you do not remember this password then you will have to reset the password or create a new account with another email address. Without the correct password or the recovery key it is not possible to connect to this account. The data stored on the Sync server is encrypted locally with a key derived from the original password before it is uploaded and only this password or a recovery key can decrypt your personal data.

• https://support.mozilla.org/en-US/kb/ive-lost-my-firefox-sync-account-information
• https://support.mozilla.org/en-US/kb/reset-your-firefox-account-password-recovery-keys

Is your intention to restore personal data from the Sync server, possibly because you reinstalled Firefox or updated your computer or otherwise lost your personal data ?

Note that Sync isn't designed to be used as a backup, but is merely meant to sync data between connected devices.

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.