Yeah, I booted the latest tails in a VM. When TorBrowser / Iceweasel starts, it takes you to https://tails.boum.org, some page on that site., which is using SSL. Immediately I entered my favorite web IP checking site, wtfismyip.com, and got that pop up message. It's just warning you you're going from an encrypted to an unencrypted connection. The part that Firefox / Iceweasel / TorBrowser / the Tails devs haven't fixed is if you enter an onion address, it IS encrypted the whole way. It's equivalent to an https connection, even though it says http. That's why I said don't worry about it, just disable it and keep in mind that when visiting clearnet sites, the exit node can potentially read anything you send over http, so don't send username/password.