I think it would suit us, firstly because it is good work experience, and secondly because we can get discounts on books and CDs.

firstly 是副词，由 first（第一）加上后缀 -ly 构成，用于列举理由或观点时表示"首先、第一点"。这种用法常见于正式或书面表达中，在口语中更常用 first 或 to begin with 来表达相同意思。