Sorry but this does not sound at all good.

Sync is not intended to be a server based backup System. The backups as such are whatever is on your multiple devices. Having said that Sync is additive it is supposed to add data not subtract it, so I do not now what went wrong here.

I did see another post recently about data loss while Sync was in use.

• What do you mean by doing reset on your computer ?
• Do you have another device whether Synced or not with similar data on it ?

If Sync has failed and you wiped the only hard disk that is your data gone forever short of the remote possibility of attempting professional forensic recovery.

Do you have an "Old Firefox Data" folder on the desktop in case you have used the Refresh feature in Firefox and have created a new profile?

If you have an "Old Firefox Data" folder on the desktop then you can recover some personal data and copy related files to the current profile folder.

You can copy files like these to the current Firefox profile folder to recover specific data.

• bookmarks and history: places.sqlite
• bookmark backups: JSON backups in the bookmarkbackups folder
• SQLite files like cookies.sqlite (cookies) and formhistory.sqlite (saved form data)
• logins.json and key3.db (decryption key) for Passwords saved in the Password Manager
• cert8.db and possibly cert_override.txt for (intermediate) certificates stored in the Certificate Manager
• persdict.dat for words you added to the spelling checker dictionary
• permissions.sqlite for Permissions and possibly content-prefs.sqlite for other Site Preferences
• sessionstore.js for open tabs and pinned tabs and tab groups

You can use this button to go to the current Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox