As is noted in the article:

Reset your Master Password if you've forgotten it

Warning: Resetting your master password will remove all of your saved usernames and passwords.

Unless you have a backup of your Profile or if you can recover old firefox data from a System Restore Point - your logins & passwords are gone.

Is this about resetting the password of the Firefox Account used for Sync ?

If you reset the password of the Sync account then it is no longer possible to decrypt the data stored on the Sync server that was encrypted with the old password and thus all data stored on the Sync server is wiped and can't be recovered.

Note that Sync isn't meant to be used as a backup and best is always to backup your personal data locally.

• https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles