I just installed Avast 2015 on a friends brand new PC (built by me. Win 8.1, 64bit.)
When trying to use Firefox, EVERY URL resulted in a “This connection is untrusted” Security Threat warning. Telling it to ignore and accept the Certificate anyway simply brought the error back up. I ended up just uninstalling Avast. Definitely bad for business.
I did happen to notice that every URL I typed auto-prefixed itself with “https://” (ie: Secured Connection.) So, for example, typing “facebook.com”, the URL became “https://www.facebook.com”. Ditto for Google & Youtube. “Internet Explorer” did not share this problem.
Disabling Avast temporarily resolved the problem, confirming it was at fault. I ended up uninstalling Avast when I couldn’t find a fix.
I use Avast on my own 64bit Win7 PC and have never had a problem, but after researching online, apparently it’s a known issue with Windows 8.1 that needs to be resolved. 