Firefox Sync <u>'''is not a data backup service'''</u>. Firefox Sync is intended to mirror changes made on one device to another device. It doesn't necessarily keep a complete or permanent copy of your data on the Firefox Sync servers. Additionally, certain account actions (like resetting your password) will erase any existing Firefox Sync data. If you used the Windows 10 reset tool, your old data may be in the <code>windows.old</code> folder that Windows creates on your hard drive. You may be able to use that folder to [[Recovering important data from an old profile|recover your old data]]. If you formatted your hard drive and the data is not on Firefox Sync and your don't have a copy or device with that data, you may be out of luck.