And sometimes, protocol aside, there is another, rather practical, reason why priests may be addressed as ‘Father [First Name]’ or ‘Father [Last Name]’. I used to live in a city with a parish blessed with four priests … as it happened, two of them were brothers (therefore sharing the last name), so they each always “Father [first name]” because if you called the parish office and asked for Father [Last Name], the question would be “which one?” Coincidentally, the other two priests shared the same first name, so they were each “Father [Last Name]”.
Then, where I am living now, there was a retired priest who frequently helped out who was some degree of cousin (close, but not first cousins) to the pastor of the parish, and they both shared the same surname, so the custom was to call them each “Father [First Name}”. The elder (retired) priest has since passed away, but the tendency is still to call our pastor “Father [First Name]” because it just seems weird to switch to what seems a higher level of formality.
Deacons–I’ve generally heard them called by “Deacon [First name]” in both of the dioceses in which I have/currently do reside.