Even though they are both adverbs, 'first' and 'firstly' are hardly interchangeable in all situations: we never say “I firstly noticed it yesterday." One might say "firstly, what are you doing in my home?" or "firstly, I hope you have insurance"—but if you want to avoid criticism, 'first' is the best bet for most ...

When should you use firstly?

You use firstly in speech or writing when you want to give a reason, make a point, or mention an item that will be followed by others connected with it.

Can you use firstly in a formal email?

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.