You use firstly in speech or writing when you want to give a reason, make a point, or mention an item that will be followed by others connected with it.
The programme is behind schedule as a result, firstly, of increased costs, then of technical problems.
Vitamin C has many roles to play in weight control. Firstly, it is needed for hormone production.
Synonyms: in the first place, initially, to begin with, to start with