I have a Mac running OSX 10.13.2 and Avast 13.4. My primary user profile on the Mac uses Avast Web Shield, and there are no issues getting to any websites.
However, when I created a new Mac user profile and enabled Avast with web shield, Firefox (on my Mac) shows many websites as “Connection not secure”, and these sites are very common (including Avast websites). When I turn off Avast Web Shield, everything works great. In searching the Internet, I found that other users have had the same issue, and have appeared to disable Web Shield to get things to work properly.
Since it works in one profile on my Mac, I have to believe that there are some settings that I can engage to make it work with Firefox (it works fine with Safari). I compared the Web Shield settings on both profiles, and they appear to be the same (including the excluded servers list).
I could not even post this entry without disabling Web Shield.
Thanks.