Sync intentionally doesn't allow you to see your data on the web: it's encrypted. If you could view it through a webpage, it could be stolen or subpoenaed, and we're not willing to take that risk with your passwords and history.
If you want to see what Sync is doing, you can add the Sync icon to the Add-on toolbar. If you really want to see what it's doing, turn on logging and look in about:sync-log!