Just found this on the Foobar FAQ
Playing certain files over HTTP doesn’t seem to work in foobar2000. It works in other players. What’s wrong?
foobar2000 uses MIME types to determine the file type of a given file. If an invalid MIME type is returned, it will cause foobar2000 to use the wrong decoder. If an unidentified MIME type is returned, foobar2000 will use the extension to determine the file’s contents. Using the extension to determine the file’s contents is a difficult thing to do in code, and is unlikely to be included in foobar2000. To fix the problem, get the server to return valid MIME types. This is a server-side problem, not a foobar2000 problem.”
Does Firefly return the right mime type?