Suppose I am enumerating reasons not to fly. Is it then correct to write/say:
Firstly, I prefer the train because I can see the landscape. Secondly, I have control over my luggage, and thirdly, it is better for the environment.
Or is it rather:
First, I prefer the train because I can see the landscape. Second, I have control over my luggage, and third, it is better for the environment.
I thought the first should be correct, but I find the second one in written texts. Which one is correct?
Both styles are used. In most genres, no-one will object to either. However, traditionally, first, secondly, thirdly etc. is used. Only pedants will insist on this usage, but it is something to be aware of, as there are many pedants. See Fowler’s Modern English Usage (3rd edition).
The Oxford English Dictionary on firstly:
Used only in enumerating heads, topics, etc. in discourse; and many writers prefer first, even though closely followed by secondly, thirdly, etc.
Burchfield in Fowler’s Modern English Usage on first: