Sorry, but Sync wasn't intended to be used as a backup media for the purpose of reinstalling the operating system. Sync was made to synchronize multiple Firefox installations on multiple devices. The big difference is that if the user reinstalls one device, another device would have the user's data should 'something' happen.

One problem that I see posted here is when the user tries using Sync with only one device - the user resets the Sync password and that wipes their data from the Sync server. A different password won't decrypt any saved data - the old password held the "key" for the encryption algorithm; a different password makes the saved data useless.