Use a plug in for most audio/video files. Then you won't need to set a
helper application, the plug in will handle them
The problem was, Firefox was using some plugins that did not handle the
media very well. There is no way to get it to change plugins. Also,
with audio files that are not embedded, I see no logical reason why the
player has to be embedded. I prefer a helper for those.
The real problem was that if Firefox has any plugin that can handle the
file, it will keep reverting to using it. If you set a helper app in
the Options, that setting lasts only for the current session. I was
obliged to disable plugins for some file content types in order to get
the helper apps to be used with them. This requires some special
knowledge of the configuration, and is not something one would expect a
person who just started to use Mozilla applications to know.