Review and audit Mozilla Sync

How safe is Mozilla Sync for Tor Browser users? The actual sync data is end-to-end encrypted, but is the metadata significant? Is it OK to use the official Mozilla Sync servers, or can they become a vector for targeted exploitation?

Also, how about safety? Can it damage your TBB profile in ways that reduce your anonymity? Do we need to/can we hack sync in TBB so that it only will sync items with other Tor Browsers and not the official Firefox?