But Firefly doesnt index the Artist-tag, (This is an assumption though I cant brows these songs by artist, but the are displayd in genre or if I serch for their song name.)
As ccrdude mentioned, I found stuff like this to happen when the id3v1 and id3v2 tags are different.
Firefly prefers id3v2 tags, and ignores id3v1 tags in favor of them. Other taggers may do thing differently. Or perhaps a tag that other applications read correctly, firefly (or rather libid3tag) doesn’t read, and so Firefly falls back to the id3v1 tag, since it doesn’t see a (valid) id3v2 tag.
I’ve only seen the second case once, but it’s possible that’s what you are seeing as well. Corrupt id3v2 tags, or unsynchronized id3v1 and id3v2 tags.