Until recent years we wrote firstly but modern, informal usage settles for first. First, he finished his task and then he ate. (Tells us when he ate.) Scholarly writing still holds to the adding of the –ly. For the rest of us, Importantly has become important. Most important, he did his job well. If we don't delete the ending, the writing sounds stilted or pompous.
* First, I want your attention, and second, I want you to smile when I pause. In the past we would have written firstly and secondly.
* Most important, listen to your mother.
Modern usage tends to strip away the –ly on adverbs.