/ ˈfɜːstli; NAmE ˈfɜːrst-/ adverb
used to introduce the first of a list of points you want to make in a speech or piece of writing :
There are two reasons for this decision: firstly ...
••
WHICH WORD
firstly / first of all / at first
Firstly and first (of all) are used to introduce a series of facts, reasons, opinions, etc.:
The brochure is divided into two sections, dealing firstly with basic courses and secondly with advanced ones.
Firstly is more common in BrE than in NAmE .
At first is used to talk about the situation at the beginning of a period of time, especially when you are comparing it with a different situation at a later period:
Maggie had seen him nearly every day at first. Now she saw him much less.