English problem!!!(URGENT)

Example:
First come, first serve : 先到先得
First floor, second floor, third floor, forth floor: 一樓, 二樓, 三樓, 四樓..
First piority: 優先選擇 或第一選擇

Firstly 通常用在一句句子的頭來引述第一要做什麼, 第一我覺得點點點...

跟住第二段就用Secondly, 第二要點點點...

So I think your first example is wrong. The second example is correct. We should use firstly in front of this sentence.

Hope it can help.

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First...and
Firstly ......
一個要用and一個唔用囉~~照你個例子睇黎

First係形容詞na~~
She is the first one to do it.
Firstly係副詞~
Firstly, we have to finish it.