The Collaborative International Dictionary of English v.0.48:

Firstly \First"ly\, adv.
   In the first place; before anything else; -- sometimes
   improperly used for first.
   [1913 Webster]

WordNet (r) 3.0 (2006):

firstly
    adv 1: before anything else; "first we must consider the garter
           snake" [syn: first, firstly, foremost, first of
           all, first off]