“Firstly” is a B2 level adverb that is used to refer to the first thing in a list that you are going to talk or write about. It is a word that candidates should be very comfortable using in the speaking, and particularly in the writing paper.
“Firstly, I believe it is extremely important for people to be aware of the damage that is caused to the environment by air travel…”
Examiner: “I would now like you you both to decide which way of working is the most productive”.
Candidate A: “Well, firstly, I think we can eliminate working in a cafe…”