There are two reasons for my decision: firstly, it's cost-effective, and secondly, it's environmentally friendly.

secondly 是副词，由序数词 second（第二）加上后缀 -ly 构成，用于列举时表示"第二点"或"其次"。这种构词方式常见于列举事项时使用的序数副词（如 firstly, thirdly 等）。