How do you associate an action with a file type in Firefox 3.0 when the file is actually in xml format?

How do you associate an action with a file type in Firefox 3.0 when the file is actually in xml format? A program that I use has its own file extension, but actually stores its data in XML format. What I want to be able to do is open these files from firefox and be allowed to associate them with the program that created them. However, instead of seeing a new file extension and asking me what to do with it, FF ignores the extension, automatically recognizes the content as XML, and just displays the XML.