Having read the books some time ago, I am under the impression that firearms are never used or mentioned in the Dragonriders of Pern series.
In support of this idea, the villains in Dragon's Dawn do not seem to have them handy, nor are they mentioned as being used against hostile creatures that would have been convenient to have them available for.
I imagine that firearms would have been among the technologies left behind as per the colony charter, but there were other technologies that were used briefly during the start-up phase after landing.
Do I remember correctly, or was there mention of firearms somewhere in the books?