I assume you are trying to print to a networked printer (perhaps one hanging off another box or even stand-alone on your network)?
If so, then you need to set up a local printer entry that points to the networked printer. The easiest (and most consistent) way is to use the CUPS administrator website.
Make sure CUPS is running locally, open your favorite browser and browse to http://localhost:631.
From there you can walk through setting up a local printer that prints over the network. Then when you go to print from Firefox, hopefully the new printer will show up in the dropdown.
As for the other entries already in your printer dropdown, I've got no clue.