Can anyone explain this clearly?

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This Connection is Untrusted You have asked Firefox to connect When I have tries to open the site yahoo following message comes. "securely to login.yahoo.com, 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 login.yahoo.com uses an invalid security certificate. The certificate is not trusted because the issuer certificate has expired. The certificate will not be valid until 12/21/2010 5:30 AM. (Error code: sec_error_expired_issuer_certificate) I Understand the Risks If you understand what's going on, you can tell Firefox to start trusting this site's identification. Even if you trust the site, this error could mean that someone is tampering with your connection. Don't add an exception unless you know there's a good reason why this site doesn't use trusted identification." Now what is the issue of "login.yahoo.com uses an invalid security certificate"? Why the certificate is not trusted? Why the certificate will not be valid until 12/21/2010 5:30 AM? Is it insecure to open yahoo ID till until 12/21/2010 5:30 AM?