The latest program update of Avast made some changes to improve the scanning of http traffic. By allowing avastsvc.exe in you (now gone) firewall, you would be allowing the new improved method of scanning. Just to be clear, it is not that "all" has changed, but only one little thing.
By changing that setting (the one I suggested), you simply went back to use the same methods that previous Avast versions used. This change is new, and some systems might have some conflicts. That's why the setting is there.
If you were safe before (and you were), then you are still safe now.
Now, Avast is a powerfull antivirus. If you are using the Free edition of Avast, then you should use some firewall, which takes care of another important layer of your security. At least use the default Windows firewall if you don't want any other tool.