So what this synced to any other devices or just one computer? If only one then reinstalling Windows wiped cleaned all data. Sync may or may not be complete as sometimes it takes time for firefox server to restore if there was a sync option set.

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

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

Did you possibly reset the password of the Sync account because you forgot the password? Resetting the password for the Firefox Account because you do not remember the password will remove all data stored on the Sync server because this password is used to encrypt your data before it is uploaded to the Sync server and only this password or a possible recovery key can decrypt this data.

You can install the About Sync extension in Firefox 72 and newer on desktop to check what data is stored on the Sync server via the about:sync page provided by this extension.

• About Sync https://addons.mozilla.org/firefox/addon/about-sync/

Close and restart Firefox to access the about:sync page added by this extension (see the about:about page for possible about pages).