Note, we STRONGLY suggest not using Avast because it has a tendency to do shady things that actually make your computer less secure (such as intercepting all your network traffic, which is why you have this error in Firefox). It's better to uninstall it, and use Windows Defender, which is free, lightweight and a far better anti-virus. Then, use Malwarebytes (www.malwarebytes.com) for monthly deep scans. Beyond that, make sure you keep Windows up to date.