The best replacement for first is firstly.

The best alternatives to first are rated as following:
★★★★★  firstly
★★★★★  third person
★★  second focal plane
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★  second language acquisition
★  second degree murder
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★  second industrial revolution
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Your alternative for first is not listed? Suggest!
Speaking about first, one probably shouldn't forget about hitting the ball.

Other search terms related to first and their ratings are:
★★★★★ hitting the ball
★★★ second
★ paying off my second
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Google Trends

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