Secure Connection to trac.videolan.org cannot be confirmed by Firefox 4.0

When I tried opening a link to a page on https://trac.videolan.org (and subsequently just that link itself) in Firefox 4.0, I got the following message:

This Connection is Untrusted

      You have asked Firefox to connect

securely to trac.videolan.org, but we can't confirm that your connection is secure. Normally, when you try to connect securely, sites will present trusted identification to prove that you are going to the right place. However, this site's identity can't be verified.

      What Should I Do?
      
        If you usually connect to

this site without problems, this error could mean that someone is trying to impersonate the site, and you shouldn't continue.

      Technical Details
      trac.videolan.org uses an invalid security certificate.

The certificate is not trusted because no issuer chain was provided.

(Error code: sec_error_unknown_issuer)