Synonyms for Firstly:

adv.

basically (adverb)
primarily.
firstly (adverb)
primarily.
first off (adverb)
first of all, foremost.
initially (adverb)
maidenly, Preliminarily, formatively, Aboriginally, initially, originally, preparatorily.

adv

firstly (adverb)
originally, primarily, initially.
first off (adverb)
first of all.
initially (adverb)
maidenly, Preliminarily, formatively, Aboriginally, initially, originally, preparatorily.

n.

arguably, Pshaw, ha, puh-leeze, what does something have to do with...?, right, Hah, of course, My. last but not least, Et cetera, lastly, and so on/forth, ...or something (like that), secondly, thirdly, second, to begin with. firstly (noun)
first of all, foremost, first off, first.

Other synonyms:

secondly, thirdly, arguably, Pshaw, puh-leeze, Et cetera. My. ha. second. right. Other relevant words:
first, to begin with, first of all, My, Pshaw, foremost, lastly, first off, thirdly.

Usage examples for firstly

  1. Firstly she was frightened. – The Vultures by Henry Seton Merriman
  2. We establish its antiquity by two arguments: firstly because everywhere in the world there are found many monuments of our ancestors; secondly, because all other societies derive their origin from this. – A Collection of College Words and Customs by Benjamin Homer Hall
  3. My advice to wives therefore is: Firstly do away with all weekly or cash payments, which are a weariness to the wifely brain. – Modern marriage and how to bear it by Maud Churton Braby