There are two reasons we can't go: firstly, we're busy, and secondly, it's too far.

firstly 是副词，由 first（第一）加上后缀 -ly 构成，用于列举事项时表示"首先、第一点"。这种形式比单独用 first 更正式，常用于书面语或正式场合中列举要点。