The other day, on OS X, I happened to start a new copy of Firefox in an account that had parental controls enabled (such as Guest user by default). It was a real eye-opener.
Parental controls alerts anytime an https connection is initiated. Basically it was impossible to keep Firefox from immediately initiating quite a number of these. I frantically tried to uncheck all the relevant preferences I could (e.g. "Block reported web forgeries").
I failed. Firefox still insisted on phoning home (maybe I missed something?). Also, it was hard to even make progress. The parental controls popup takes focus and demands an administrator's approval to proceed. By the time I could dismiss the popup, Firefox tried to phone home again and a new popup appeared.
Sheesh.
Here are just some of the sites that Firefox immediately accesses:
self-repair.mozilla.org
snippets.cdn.mozilla.net
search.yahoo.com
location.services.mozilla.com
www.mozilla.org
tiles.services.mozilla.com
safebrowsing.google.com
aus4.mozilla.org