Looking through the firewall logs in isolation or after a long time - it is going to be hard to know the context of why the connection was made.
The avastSvc.exe is the main avast server and controls the shields, etc. When you are browsing the internet the web shield will be scanning the content - to do that it redirects the browser calls through avast so that they may be scanned before allowing the browser to view the content.
All of this is done using the avastSvc.exe, so it isn't shown as a request from your browser or the web shield.
Can you pin down roughly what you were doing at the time this appears in the firewall logs ?