Sync isn't intended to be used as a backup media for the purpose of formatting a device. It's intended purpose is to synchronize Firefox data across multiple devices.

Firefox Sync stores one version of bookmarks, no backups or multiple copies. Multiple bookmark backups are stored in the local device but those multiple backups aren't sent to the Sync server, only the newest copy is saved on the server. And the synchronization is done via changes per session; changes to bookmarks are for just the changes - removal or additions.