I think the mime type might be wrong on the server - I get text just loading the file using:
http://homepage.ntlworld.com/alan.go...testnews.shtml
But if I save it locally and open that it opens ok, though obviously saving it changed the meta data, Firefox just ignores the <!--#include file="file.htm" --> as any decent browser should.
I don't think it is an issue with ssi working on the server as the rest of the html should still display - something else is causing the browser(s) to think it is a plain text file.
Bear in mind I know almost nothing about web servers and html.