An Encylopedia Britannica Company

firstly

1 ENTRIES FOUND:
firstly /ˈfɚstli/ adverb
firstly
/ˈfɚstli/
adverb
Britannica Dictionary definition of FIRSTLY
used to introduce a statement that is the first in a series of statements
Firstly is chiefly used to begin lists and is usually followed by secondly, thirdly, etc.
◊ In U.S. English, first is more common than firstly in this use.
compare lastly, secondly, thirdly