In writing, first is much more common than firstly and is probably the best bet for most situations, even if the only reason is that a reader may wonder if firstly is proper and be distracted by this word choice.

Is firstly formal or informal?

You can use either first or firstly as an adverb to introduce a statement that is the first in a series of statements. The use of firstly is more formal. Similarly, you can also use second, third, etc. instead of secondly, thirdly, etc. to refer to further points or statements.

Should you use firstly in an essay?

You should use first, second, and third to show textual enumerations in your writing. Many authorities prefer first, not firstly, even when the remaining items or points are introduced with secondly and thirdly.