I have the same problem:
Avast 11.5
Firefox 43.0.3
OS X 10.6.8
When trying to access yahoo.com, google.com, wikipedia.com, duckduckgo.com, etc. all give this error from Firefox:
THIS CONNECTION IS UNTRUSTED
TECHNICAL DETAILS
duckduckgo.com 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)
If I disable Avast Web Shield, everything works. All these sites work in Safari 5.1.10.