First a program update is going to trigger several firewall confirmations, firstly avast.setup otherwise you won't be able to update, check your firewall allows avast.setup if you find an entry for it is it blocked, change it or delete the entry and do a manual update, your firewall should ask permission.
Then any changed modules, like ashWebSv.exe, ashMaiSv.exe, etc. these updates will/should have your firewall ask permission, grant it.
When your firewall pops-up a window please take a little time to read what it is about before automatically clicking NO or worse still YES.