The problem seemed to be directly linked to AVG and Avast antivirus programs clashing with Firefox 65. Specifically, the error message appeared when an HTTP website was visited. This was happening on all sites, even major services. Users reported seeing ‘Certificate is not trusted because the issuer is unknown’ and that ‘The server might not be sending the inappropriate intermediate certificates’. Mozilla reported the problem on its bug report portal. The company says the HTTPS filtering feature found in AVG and Avast is stopping users from visiting HTTPS websites. It is worth noting that AVG is owned by Avast.
Solving the Problem
One obvious way to negate this problem is to simply not use those two antivirus services. However, plenty of users are tied into those services for various reasons. With that in mind, Mozilla says users should pause automatic updated on Windows. Avast has released an update for its antivirus and AVG. The update turns off HTTPS filtering in Firefox 65, while the feature is still available on other browsers. We guess the company will fix the problem and bring HTTPS filtering back to Firefox at some point. For the time being, pausing the feature gives the company time to work on a fix.
