I have been trying to add an add-on on firefox but FF just show this error message. I checked my time setting and its ok. I cant also open other major sites like twitter. I already have tried the solutions indicated in here: https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message. But nothing works for me. What should I do? Please help. Thanks This Connection is Untrusted You have asked Firefox to connect securely to services.addons.mozilla.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. This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox only connect to it securely. As a result, it is not possible to add an exception for this certificate. services.addons.mozilla.org uses an invalid security certificate. The certificate is not trusted because the issuer certificate is unknown. The server might not be sending the appropriate intermediate certificates. An additional root certificate may need to be imported. (Error code: sec_error_unknown_issuer)