Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
firstly, first (advs.)
Both firstly and first are adverbs, and some have argued that firstly (which has been around since the sixteenth century) is unnecessary and ought to be avoided. But both are Standard, even though firstly may seem a bit old-fashioned. Remember though that in a series of items, you should stick with the form you begin with: if first, then second, and so on; if firstly, then secondly, etc.
