title: Adding custom content-types to Firefox application handler?
body: <p>In Firefox, whenever I request a page that returns a Content-type of "application/json", Firefox saves the file into /tmp but then says "/tmp/json.php could not be opened, because the associated helper application does not exist. Change the association in your preferences."  Well, I go into Edit > Preferences > Applications, and there's no content type listed there that seems to have anything to do with application/json.</p>
  
  <p>Is it possible to add an entry for this type?  (Or for a random custom type I make up, like "foo/bar"?)  I see a file called mimeTypes.rdf which looks promising.</p>