Hi Viktoria. Firefox Sync is <u>'''NOT A DATA BACKUP SERVICE'''</u>. It wasn't designed for that purpose, so it doesn't always function that way. Sometimes there's not always a complete or permanent copy of your data stored on the Firefox Sync servers. Additionally, some account actions - like resetting your password - will delete data that's stored on Firefox Sync. That said, did you reinstall Windows using the Windows reset feature or did you completely repartition your drive using a Windows installation disc or USB? If you used the Windows reset feature, that usually places your personal files in a <code>windows.old</code> folder on your hard drive. If that's the case, your old Firefox data may in there.