There's an hour long video discussing the measures taken to protect the user's privacy, given by Ben Adida, one of the developers. It's 12 months old, but I believe the protocol hasn't changed enough to make the information out of date.

https://www.youtube.com/watch?v=QDSh3osE4GQ

Also, see this blog post:

http://identity.mozilla.com/post/7899984443/privacy-and-brow...