The question First, … Secondly, … &c, posted two years ago, asks whether there is any basis besides tradition for preferring first to firstly. Five days after it was posted, the question was closed as a duplicate of “Firstly, secondly” vs. “Firstly, lastly” when listing just two points, which asks whether it is acceptable to use lastly in place of secondly in a situation where a person is introducing the second item on a list of just two items. The question doesn't ask about "first, second, third" versus "firstly, secondly, thirdly"; indeed, it doesn't mention first, second, or third at all.
The close reason states "This question already has an answer here" and points to the secondly versus lastly question—and indeed the sole answer to that question does glancingly address the question of "first, second, third" versus "firstly, secondly, thirdly" in its final paragraph:
Oh, and drop the "-ly" ending after first, second, third, etc. It isn't necessary, and it does not make you sound more erudite (in my opinion).
But it seems to me that this offhand remark does not come close to justifying the assertion that the question about first versus firstly "already has an answer here." I ask that you consider reopening the First, … Secondly, … &c question.