Should I (assistant professor) call undergraduates by first name or Mr/Ms There are lots of questions about how undergrads should address professors, but this is about the opposite. Context is North American research university.

My instinct and personal preference is to ask undergraduates to address me as Professor Lastname or Dr. Lastname until I let them know otherwise. And I have no problem with explicitly saying this in my syllabus for a class.

But then sometimes I think "actually why aren't I addressing students as Mr./Ms./Mx/(or some honorofic) Lastname".

Should I do so? Do you do so?
It is disrespectful to demand more respect from your (adult!) students than you give them. So you should either both go my first name or both by last name. At least this is true in by culture (Central Europe) - I would think it is the same in Northern America.
So, unless it is a cultural thing everyone does in your area to call younger people by their first name, by respectful and call them as you want to be called.