Since there are more and more peolpe who have the same problem while connecting to his NAS with firefox3 but shows "Secure Connection Failed" message.
Therefore I provide the security exception setting for a possible solution with this problem.
1) When you connect to DS through https in Firefox 3, the browser won’t let you pass if the certificate is invalid.
2) For solving this, click “Or you can add an exception” from the error page, then click “Add Exception”.
3) In “Add Security Exception” dialog, press “Get Certificate” button. The “Get Certificate” will override how firefox identifies the site.
4) Press “Confirm Security Exception” button to add the site to the exception list and reconnect to the site.
5) Now you can access DS management UI through https in Firefox 3.
Hope this is useful for you while having the same problem.