Avast & Firefox problem

I have discovered an issue with Avast & Firefox. This may have already been discussed but my search couldn’t find any answers.

When I open Firefox and type in Google in the search bar I get this … (see attached screenshot).

If I disable the Avast “Web Shield” it goes through the search to Google no problem.

With the Web Shield enabled I cannot log on to Google.com

This does NOT happen in Safari with the Web Shield enabled.

Wondering what’s wrong, and if there is a fix with the web shield enabled?

OSX 10.13
Avast Security for Mac 12.9
Firefox 56.0

Tell Mozilla that they should accept the certificate from avast.

Probably related to: https://forum.avast.com/index.php?msg=1423353
Anyway, I have forwarded it to the Mac guys for you… Cheers