AFAIK, they can only be created at the command line, not configured. If you want to do things like change default settings or install extensions from the Firefox Add-On store, you can't really do that at the command line.

You can do that by mucking around in the user.js file and manually adding .xpi files to the extensions/ subfolder, but that's probably stretching the definition of "done at the command-line" since most people aren't creating Puppet modules to manage Firefox profiles.

Perhaps someone knows an easier way to do this, though.