thanks for your answer
2. the web shield doesn't provide any firewall functionality so yes you continue to use your firewall.
my question is the opposite.
if i have a firewall is it necessary to activate the web shield of avast?
Sorry I didn't read the question very well.
But, Yes I would say you do need to have the web shield enabled, your firewall won't stop legitimately downloaded http traffic. You didn't say what firewall you use, I have Outpost Pro and that has a number of plug-ins but it isn't specifically looking for malware.