Is it acceptable for me (an undergrad) to call professors and other research professionals by their first names?
In all the years at the university, I only called the professors and lecturers by their last name and a doctor prefix. I also used to call them Professor + Last Name.

But never by their first names as far as they were older than me, they were professors and more important, it was not a custom in the universities I studied to call professors by their first names only (it was somehow impolite to call a professor by his first name only, even by other professors or colleagues).

It usually goes back to the custom. Sometimes the professors themselves prefer to be called by their first names. See how others call professors and see if it is polite or not. But my prefer is always Doctor + Last Name.