A screenshot of the popup/notifications would be helpful.
Avast Free (if that is what you installed ?) doesn't have a basic firewall. I suspect that you are seeing a notification that you only have a basic firewall (the windows one), effectively a promotion to but Avast Premium Security that does have a more comprehensive firewall.
These promotions/ads for other avast paid products is the cost of having avast free (as I do). I don't like them as I don't like the methods employed to push them, but I just ignore them.
So if you have avast free, these popups are unrelated to blocking friends using your minecraft server, so again a screenshot of any error screen, etc. would be helpful.