On every web site I try to access in Firefox I get the Your Connection is not Secure notice. I think the Mac is now infected with malware. Right before this began I was on a web site and a popup appeared, which I now believe was a fake Flash update popup. I clicked OK thinking I was getting rid of it but instead I believe it installed malware. What to do? I have already tried Malwarebytes and it removed a few things but the problem still exists.
The owner of [badsite.com] has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.
This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox may only connect to it securely. As a result, it is not possible to add an exception for this certificate.
Continuing on, when I click Advanced I see
[web address] 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
And when clicking on the error here is what it says: