What is the difference between first and firstly?

Answer 1

First is commonly used and firstly is used when you want be more formal.

Both are used to show order.

Examples:

First, she has to go to the doctor.

Firstly, you must go to the doctor.

Her first name is Shelly.

Answer 2

First is an adjective. It is a part of speech used to modify a noun, pronoun, or another adjective.

Firstly is an adverb. It is a part of speech used to modify a verb or another adverb.

While both can often be used interchangeably, doing so without fully understanding grammar can create ugly and ungainly sentences.