'''''"... there was some kind of migration in sync system."''''' Almost two years ago ''[April 2014]'' an improved Sync 1.5 version came out with Firefox 29. Information about upgrading Sync was provided inside Firefox after the user updated Firefox, but too many users delayed updating beyond Firefox 28 due to the new User Interface or missed the seeing the "update Sync" message. ''User error - IMO.'' '''''"But today! Fresh linux install, everything fresh, installing firefox ..."''''' Sync wasn't intended to be used in lieu of a proper backup when doing a fresh OS installation. Sync is made to synchronize Firefox data across multiple devices, where at least one device would have the users unencrypted data while the user is fixing a device or installing a new OS. A mistake made when connecting back to Sync ''[or a glitch with Sync itself]'' is easily fixed by resetting the account password ''[which resets the account on the Sync server for a fresh start]'' and then sync'ing the original, unencrypted data from another device to populate the Sync server again. Then connecting the new Firefox installation in the new OS to Sync synchronizes that device. The '''most common mistake that users make''' when attempting to use Sync as a "cloud" for backup data storage, is that '''they inadvertently reset their Sync password''' which makes the encrypted data in their Sync account on the server worthless - a "fatal mistake". The Sync password is used to create the '''encryption key''' for their data while that data is off of their devices; without the original "key" that data can't be decrypted - it was rendered useless when the "key" was changed. IMO, the level of encryption used by Sync is higher then necessary for data saved in a web browser, but "you" can't fault Mozilla for providing too much security given the number of publicized worldwide data breaches that have occurred in recent years.