Your server is not sending the correct mime type for the file.
It send Content-Type: text/plain
The HTML5 video may play in Safari, Chrome and IE 9 but not Firefox or IE 7-8. If you fix the MIME-type issue, it will play in Firefox.
If you’re using the Apache web server or some derivative of Apache, you can use an AddType directive in your site-wide httpd.conf or in an .htaccess file in the directory where you store your video files. (If you use some other web server, consult your server’s documentation on how to set the Content-Type HTTP header for specific file types.)