先ずは英語で
firstly

英語の定義

先ず
1. adv. first of all
そう？先ずメイド服の色が違うし… - Really? First the color of the maid costume was different...
2. adv. anyway
3. adv. about; roughly; just about
4. adv. (with a negative verb) hardly; not really
5. adv. Usage: * (ja-uk)

firstly
1. 副詞. (formal)最初に。はじめに。まず第一に。

firstly
1. adv. (formal) In the first place; before anything else; first.

first of all

first of all

first of all
1. adv. (sequence, idiomatic) Firstly; before anything else.

first
1. 形容詞. （oneの序数詞）第一の、一番の、最初の。
2. 形容詞. （否定文中で）ほんの少しの（ことがらも～ない）、（について）全く（～ない）
3. 副詞. 第一に、最初に、まず、先に
4. 副詞. 初めて
5. 副詞. いっそ、むしろ
6. 名詞. 第一位の人又は物。
7. 名詞. （月の第1日目）１日（ついたち）
8. 名詞. （自動車の変速機で）第一速。
9. 名詞. 初めてのこと、前代未聞のこと。
This is a first. For once he has nothing to say.
これは初めてだよ、前はそんなことは言わなかった。
10. 名詞. （英　口語）一流の地位。
11. 名詞. （野球）一塁。
There was a close play at first.
ファーストでクロスプレーがあった。

first
1. adj. Preceding all others of a series or kind; the ordinal of one; earliest.
The first day of September 2013 was a Sunday.
I was the first runner to reach the finish line, and won the race.
2. adj. Most eminent or exalted; most excellent; chief; highest.
Demosthenes was the first orator of Greece.
3. adv. Before anything else; firstly.
Clean the sink first, before you even think of starting to cook.
4. n. The person or thing in the first position.
He was the first to complete the course.
5. n. The first gear of an engine.
6. n. Something that has never happened before; a new occurrence.
This is a first. For once he has nothing to say.
7. n. (baseball) first base
There was a close play at first.
8. n. (UK, colloquial) A first-class honours degree.
9. n. (colloquial) A first-edition copy of some publication.
10. n. A fraction of an integer ending in one.
one forty-first of the estate
11. n. (obsolete) Time; time granted; respite.