What does "firstly" mean?

In the first place; before anything else; first.

What is a synonym for "firstly"?

Common synonyms include: first, ab ovo, chiefly, especially, first of all, first off.

/ˈfɜɹstli/ adv

adv ·Rare ·Advanced level

Adverb

1
In the first place; before anything else; first. formal, not-comparable
"Firstly, I continue to base most species treatments on personally collected material, rather than on herbarium plants. […] Secondly, I continue to base my concepts on intensive study of a limited suite of collections, rather than superficial study of every packet that comes to hand."

Adverb

"Firstly, it is a matter of justice."

Coined before 1475, from Middle English fyrst-lyk, equivalent to first + -ly.