The best replacement for firstly is first.

The best alternatives to firstly are rated as following:
★★★★★  first
★★  first of all
★  first grammar
★  first off
  firstable
  at first
  akamai

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Speaking about firstly, one probably shouldn't forget about secondly.

Other search terms related to firstly and their ratings are:
★★★★★ secondly
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