Not via Sync. You would have to have a local backup to be able to restore your data. Maybe a low level search on the hard drive with an undelete utility could still bring up some data, but this is a complicated process and you can't be sure if clusters haven't been reused in the meantime. ----- The Sync service uses a sync key to encrypt data locally before uploading This Sync key is generated internally from the password of your Firefox account, so you do not need to worry about it. Without the correct password it isn't possible to decrypt the data stored on the Sync server because the correct Sync key can't be generated. The Sync key changes when you change the password of the Firefox account and you lose all data stored on the Sync server when you change the password of the Sync account.