Both the fired bullet and the dropped one will hit the ground at the same time. That is unless you want to get extremely technical about it. I remember this from Physics in college. Try this experiment. Grab something you don't mind dropping. It shouldn't be round or able to roll easily. Now take your object and hold it to your waist. Walk as fast as you can and let the object go in mid stride. You will find that it lands right next to you feet as you are walking. This shows that horizontal movement and vertical movement are independant. While it's hard to believe, both bullets will hit at the same time.

Now, if you want to get crazy with it, you can ponder the following. Since the earth is round, the shortest distance to the earth is right where you are standing. As you move further from that point, the distance increases. For example, if you drop a ball from right in front of your nose, it will travel a shorter distance than if you dropped the same ball from th same hight, but at arms length. Not much farther, but it is indeed a longer distance to the ground. Now considering these facts, if you want to be technical; the fired bullet will hit after the dropped one, but not by that much.

If you fire a bullet at approx 27,000KMPH it will essentially go into orbit and never land.