Referring to professors by their first names, when giving a talk,
Some professors are comfortable having students call them by their first names. You should wait until individual professors let you know that this is acceptable to them before you do so.

However, others are not comfortable with this. So, to be on the safe side, I believe that it is best to refer to them by their formal title and last name during a public talk. Many professors do this with their peers during public talks, even if members of the audience are their good colleagues. Plus, you won't run the risk of mistakenly refer to some professors by their first name and others by their formal name, which could be viewed as disrespectful.

I also caution to graduate students that even when they feel comfortable calling individual professors by their first name, they should refer to the same professor by their formal name when talking to others. It's usually best to err on the side of greater professionalism.