I always address parents in a formal manner, so that's how they address me, as well. There have been parents that I have come to be on a first name basis with (i.e. the ones I'm usually calling/emailing every day), but that doesn't happen too much. In terms of other staff members in my building addressing each other--we all refer to each other by our first names unless students are around. Sometimes we do address each other Mr. __, Mrs. __, etc. outside of school time, but that is more in a jovial manner than a formality.