How should a postdoc address senior research staff and faculty members in the USA? What is the etiquette for addressing senior research staff and faculty members in an academic setting in the USA? A postdoc is no longer a student but at the same time he or she is (in general) not at the level of senior research engineers/scientists and professors. How should he or she address them ("FirstName" or "Dr. LastName") so that proper respect as well as confidence in one's "above-the-students" position is maintained?

Does it depend on the University, Lab group or the level of experience one has acquired before joining the present postdoc position?
When I was a grad student, my initial preference was to call faculty "Doctor" or "Professor".  At some point (about halfway through, I think) one professor said to me: "If you want them to see you as a peer, you should call the professors the same way that they call each other."  At my university, all the professors called each other by their first names (it didn't matter if they were assistant, associate, or full).  Since I planned to make a career in academia, I really did want to be viewed as a peer.  So I started calling them by their first name (and no one ever told me to do otherwise).