I don't know what it's like these days, but when I was being trained as a pilot in the 1960s, I (an officer cadet) had to address my instructor ( a sergeant pilot) as 'sir' when we were in the air - he was captain of the aircraft. He had to 'sir' me at all other times.
Back to the topic.
As we have seen, the use of Mr/Ms/etc followed by a first name is normal in some contexts. However I would recommend that learners do not do this unless they have been told to. The use of forenames (without a title) , even to older people and/or those in a position of authority, is far more common that it was in my youth, but is not universal. If somebody is introduced to you by their first name, then it is pretty safe to assume that that is how you address them.
If they are introduced as First name, Surname, then the situation is not so clear. If you are in doubt, particularly if they are older than you and/or in some superior position, you will never cause offence by using a title plus surname.