Can't connect with the just downloaded version 66.0.4 to virtually any site - not to Google, not to NYT, not even to Mozilla - Firefox won't accept ANY certificate! Following errors appear (examples): "Did Not Connect: Potential Security Issue Firefox detected a potential security threat and did not continue to addons.cdn.mozilla.net because this web site requires a secure connection. What can you do about it? addons.cdn.mozilla.net has a security policy called HTTP Strict Transport Security (HSTS), which means that Firefox can only connect to it securely. You can’t add an exception to visit this site. The issue is most likely with the web site, and there is nothing you can do to resolve it. If you are on a corporate network or using anti-virus software, you can reach out to the support teams for assistance. You can also notify the web site’s administrator about the problem. Or: Warning: Potential Security Risk Ahead Firefox detected a potential security threat and did not continue to www.nytimes.com. If you visit this site, attackers could try to steal information like your passwords, emails, or credit card details. What can you do about it? The issue is most likely with the web site, and there is nothing you can do to resolve it. If you are on a corporate network or using anti-virus software, you can reach out to the support teams for assistance. You can also notify the web site’s administrator about the problem. The explanation is as follows: Someone could be trying to impersonate the site and you should not continue. Web sites prove their identity via certificates. Firefox does not trust www.nytimes.com because its certificate issuer is unknown, the certificate is self-signed, or the server is not sending the correct intermediate certificates. Error code: SEC_ERROR_UNKNOWN_ISSUER No problems whatever with other browsers. I'm sending this with Safari.