Firefly will ignore songs for a number of reasons.
1. They're DRM protected
2. The file type is not listed in Firefly's configuration
3. Firefly does not have permission to read the files / directory
If the songs were downloaded from iTunes Music store you will have to burn a CD & then rip the CD to remove Apple's fairplay DRM protection
If the file type is not listed in Firefly's configuration, add the file extension & manually start a scan
File permissions, all files / directories must be readable by everyone as Firefly runs as a service which means that it doesn't have the same access permissions as you do. This mostly applies to Linux boxes & network shares but occasionally crops up in other situations.
Aggressive scan simply does a thorough scan of a music file to ensure that the song length in the database is the actual songs length.
If your music is ripped as a CBR file then you could set the scan type to "normal" (i.e. 0).
If you have VBR music files then you'll need a more aggressive scan type.
You can't change the location of the iTunes library file.