Well, this doesnt sound bad.
There have been wishes for playlist management like in iTunes, where one can select which playlists to share, and which not.
Firefly currently shares all playlists.
I cant think of a workaround for your problem so far, other than disabling m3u scan, or renaming this playlists to PLS or any other format that winamp recognizes and firefly doesnt (if there is such a format)