Mozilla Sync isn't meant for backup it's to sync between device to sync server back to device. What you did was to wipe clean any data on the computer that could've been found and doing a sync replaced the data that was there to resync with another device that would've been still connected to the sync server. But now it has been replaced with a new login and synced with that data which probably it didn't have data so the data was written over any previous data.