Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

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