In theory "first" is the better usage, but both are widely used by native English speakers, most of whom wouldn't be aware "firstly" is regarded by some as a substandard usage.
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Catherine Demorvanmost of whom wouldn't be aware "firstly" is regarded by some as a substandard usage.Uhm, really? I was taught 'firstly'', 'secondly' etc. are more formal, not substandard, and I was also taught they are seldom used in AmE.
Catherine DemorvanEg Guardian style guide:Well, thanks for that, but it doesn't mean it's "substandard," does it?
first, second, third
rather than firstly, secondly, thirdly, etc; spell out up to ninth, then 10th, 21st, millionth