Firefox is open, high quality and user friendly.
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
I don't think there is a rule.
So you could argue that they are not all that user friendly.
They are user friendly they just represent the usages of the type of user using them.
This is something I see said a lot, constraining an interface to your average completely new user who knows nothing makes it very difficult to have that interface adequately serve people with more experience, you load up the benefit on the front side and trash the backside in response - that said some powerful interfaces are just objectively bad but the tools utility overrides that.
The software for making a photo collage for Grandma and the software for controlling replication across a hundred database nodes can be radically different and still be user friendly.
Lots of software tends to assume that the user is a new user but the problem then is that a new user rapidly becomes a not-new user and those things that helped initially now hamper productivity (cough Clippy cough).
I'm not saying everyone should have a Symbolics terminal and that requiring lisp knowledge should be mandatory so much as lots of people say open source isn't "user friendly" when what they mean is it's not particularly friendly to new/inexperience users coming from outside.
http://rippedwire.sourceforge.net/images/hbgtk-video.jpg for example, to a new user that is very unfriendly but if you use it frequently and or understand what you are trying to do that kind of no-nonsense interface is user friendly.