What version of avast are you using, the latest regular release is 4.8.1229 ?
Well if it truly were the web shield then it wouldn't be every few days, it would be every time.
avast doesn't block, but alerts to infection so again if the web shield were the true culprit you would be getting alerts and not only that it would be blocking all http pages, it isn't selective unless you add google.com to the URL Blocking list and in that case you would get a message that it was blocked because of URL blocking.
When there is some kind of unexplained block and as I said the web shield doesn't block, it is down to some sort of conflict with another application, disabling the web shield removes one party in the conflict so no conflict, but what it doesn't do is confirm web shield is the true cause.
Check this, although this too would be a permanent not intermittent problem - HOSTS file redirect - 127.0.0.1 check your HOSTS file using notepad or a text editor of your choice, C:\WINDOWS\system32\drivers\etc\hosts or do a search for HOSTS to find it if not there.
http://en.wikipedia.org/wiki/Hosts_fileWhat security add-ons do you use in firefox ?
Google operates a local redirect so if I click on a link to google.com.like the one in your post it will redirect to my local google.co.uk page (see image). So there might be something in this redirection that is being checked by a security add-on and the web shield causing a conflict.
