Ну вот, я всё ждал, когда же оно мне само попадётся. Попалось, наконец.
Словарная справка:
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.
А я иногда очень старомодно выражался, оказывается :)