[1] With proparoxytone accent, the compound has a passive sense. This usage occurs in the Septuagint and the New Testament.
[2] With paroxytone accent, the compound has an active sense, like theotokos itself. This form and meaning is attested from Homer onwards (and generated a gloss in the scholia to Iliad 17.5), but not (apparently, but see below) in the Septuagint or the New Testament. See LSJ entry at web address 1.
The following information was contributed by Jan Krans:
Isidore of Pelusium discussed the interpretation of prwtotokos in Colossians 1:15, proposing to shift the accent to the penultimate and thus making it have an active sense (his Epistles III, 31 is dedicated to this subject -- see PG 78, cc. 749-754). His intention clearly is to combat the 'heresy' according to which Christ is only created (albeit first). Also interesting is that Erasmus either was acquainted with this entry in the Suda, or knew Homer, Iliad 17.5, and revived Isidore's interpretation in his annotations on the New Testament (1516).