mp3 tagging is a minefield. Like all metadata, one has to balance obsession with detail against ease of acquisition.
Some Firefly clients are pickier than other about tagging. Regrettably, some of the music I get from emusic has bad characters in the tags, which throws the players right off. Finding the problem files is the majority of the problem – here’s a method that at least helps:
wget -O- http://host:port/rsp/db/1 | perl -pwe 's,<(?![/\?]),\n<,g;' > firefly.xml
xmllint --noout firefly.xml
You’ll likely get a few lines like:
firefly.xml:463415: parser error : PCDATA invalid Char value 65535
<title>�Singing in the Bathtub</title>
In this case, the song title tag has some junk characters in it that you’ll need to fix. After cleaning up the tags and rebuilding the database, try this process again to see if you’ve caught all the errors.