I just updated the Free version to version 21.10 through the GUI.
Before doing so, I looked to make sure that I knew how to remove the firewall, if necessary, following the advice in this thread. Surprisingly, my version 21.9 told me that the firewall was already available, but that it wasn't installed. Surprisingly because I had looked when 21.9 was first installed here and the firewall wasn't available then, so the option had been activated sometime subsequent to that installation.
After the update, the message was the same as before that the Avast Free firewall was available, but not installed.
OK, so the update went well, and I don't have the new Avast firewall installed. But it is available, so my question is what advantage would there be over using the Windows 10 default firewall?