Hi, we are working on a fix, will be released later today.
As a workaround, you can temporarily disable TLS1.3 in firefox settings.
1. In the address bar enter about:config
2. Search or scroll to security.tls.version.max
3. Change the value to 3
Done.
This will limit the TLS version to 1.2, the default for Firefox previous version and other browsers at this moment. I'll update here when an Avast fix is ready. Undoing this configuration change is easy as well, just follow the same steps and change the value back to 4.