first [adverb] [not gradable]First(ly) is used when referring to the first thing in a list.There are two very good reasons why we can't do it. Firstly, we don't have enough money, and secondly, we don't have enough time.He studied economics firstly because he enjoyed it and secondly because he wanted to get a good job.
FIRSTLY
Meaning of FIRSTLY in English
Cambridge English vocab. Кембриджский английский словарь. 2012