Firefox Sync is <u>'''NOT A DATA BACKUP SERVICE'''</u>. It was not designed to be one and doesn't always work when used as one. Additionally, some account-related actions (like resetting your password) causes existing Firefox Sync data to be erased from the account. That said, all hope may not be lost. If you used the Windows 10 reset feature, your old Firefox data is probably saved in the <code>Windows.old</code> folder that's on your hard drive.