What is Avast Firewall? Well It's just a firewall for online applications. It shields you from certain types of web traffic as well as a variety of other threats. It does not provide the same level of security as a traditional network firewall. But What if it stops working one day? It's happening because many avast users were concerned from last week that why is avast unable to start firewall. It can be prooved very dangerous for your PC! So just update or Reinstall your Avast Antivirus or disable Windows Defender!