Which regions are you looking at? Europe or Asia? China had firearms before the Europeans did and used them in conjunction with melee units, Japan under Oda Nobunaga and Toyotomi Hideyoshi also made very effective use of firearms after the Portuguese introduced them also in conjunction with melee units. So at least for East Asia, there was a transition period between pre-firearm and post-firearm, though it seemed they never transitioned out of it. I went over firearms in Japan in this thread:

Did Oda Nobunaga invent the volley fire system?