There are two very good reasons why we can't do it. Firstly, we don't have enough money, and secondly, we don't have enough time.

Firstly 是副词 first 加上后缀 -ly 构成的，first 意思是"第一、首先"，-ly 后缀表示副词形式。firstly 用于列举事项时表示"第一点、首先"，通常与 secondly（其次）、thirdly（第三）等词配合使用。