Within the context of this passage, "firstborn" cannot mean "first created." This is true for several reasons:

1)  Immediately after Christ is called the "firstborn over creation," He is said to have created ALL things: heaven and earth, visible and invisible, principalities and powers. If He created all things, He must not have been created.

2)  Christ is also called the "firstborn from the dead." However, He was not the first one to rise from the dead. In the Old Testament, a widow's son was raised from the dead at Elijah's calling. Also, Christ raised Lazurus from the dead. Therefore, "firstborn" does not seem to have to do with time.

A better definition for "firstborn" is "primary one." In this sense, firstborn has to do with position, not time. This seems to better  fit the context of the passage as Christ is also called "the head of the body" and it says that  "in all things He may have the preeminence."