I had to look up several examples.Firstly, secondly, etc. are not real words. They're probably in the dictionary now because English is a democratic language that is not controlled by a government department or an academy of scholars, but this is bad English and you should not learn to speak or write that way.This is too informal. In a work of scholarship, just say "first."Yes, as Aqueous pointed out, this is the correct way to write it. The series of words first, second, third, etc., function both as adjectives and adverbs. There is no need to add the -ly inflection. "At the office this morning, Charles arrived first, Susan arrived second, and I arrived last.""Firstly" is not just "not formal." It is wrong! Don't ever use it. As I have explained to another member, when you're using someone else's language, you should strive to use it more correctly than they do. And you misunderstood the article. It is not just "firstly" that is wrong. The entire series is wrong. There is no such word as "secondly," "thirdly," or "six hundred thirty-fifthly."
