I'm certainly not a hands on firewall user, Outpost was/is very configurable, but for the most part it has a flexible almost friendly user interface.
On this win10 acer laptop I'm using the default win10 firewall, but the Thunderbird option to block external content, is a saving grace. I too still prefer client based email to webmail although I have a gmail account it is more of a backup than a primary option.
I would agree that it may be time to get in touch with avast support
