There was a topic (
https://forum.avast.com/index.php?topic=167577.msg1192147#msg1192147) about this, but the answer was not very helpful, so may be is better to elevate it again. I have MacMini (2015, El Capitan 10.11.1) with FF (42.0) and Avast (11.3, VPS 15111300). After fresh installation of FF it was not capable to open any page which uses https by default or is redirected to (google.com, mozilla add ons page) - always the warning page with bad certificate (sec_error_unknown_issuer) has been displayed and there was no possibility to add an exception. I tried to do it manually inside firefox, but it did not help. I also found out that Avast installed its own server certificate apparently to be able to listen the traffic and see the FF one. This certificate is self-signed. Now if the webshield is active, https pages with unknown cerrtificates to avast, which are pretty much all of them, will never open; once the webshield is deactivated, everything in FF is working normally and pages are opening. Safari is not affected despite I found the avast certificate in key chain. I have two questions:
1. Is this behavior feature or bug - please do not respond "you should not open unknown pages, because..", I do not believe, that google.com is scum or phishing..
2. How and why is avast certificate imported without my consent to FF?
Thanks