you need to ensure that you have FF pointing to where your itunes .xml file is at. If you have your music stored someplace other than the default itunes location the important point to bear in mind is not so much that place but where the .xml file is. The .xml file contains the information needed to pick up your playlists and serve them to FF. So do a search for it (if you're not sure where it is) and in the configuration page of FF web interface have FF point to that location. Then rescan and you should be set.
You can check that FF has found the playlists by going into itunes and expanding your Firefly entry in the Source column on the left side of the itunes interface. You should now see them there.
Hope that gets you set, Andy
G5, OS X 10.5.8, 3 x M1000, iTunes 10.4, slimserver 6.2.1/alienbbc (now redundant), firefly svn 1586, uNSLUng 6.8