I have the same problem too, and I’m still doubting that iTunes is making it more difficult to set Firefly that way.
Nope, you’re right… the more I thought about it, it really has nothing to do with iTunes.
When sharing, iTunes is just a dumb(ish) terminal… simply showing the tunes and playlists that are in the library that is shared.
There’s really no way that the library share can force iTunes to default to a particular playlist or directory.
So, really, the solution has to be on Firefly’s side… only sharing those tunes that I’m interested in sharing. As Lori said, that’s the easy part. Simply tell Firefly to look in certain directories for tunes to share. But the piece(s) that’s missing is the “you’ve already indexed this other directory, you don’t have to index it again” or “keep this directory in your index, but don’t actually share the tunes within” functionality.