You didn't say what firewall you used, but Zone Alarm has in the past forgotten allocated permissions even though the list said permission was granted.
There hasn't been a program update for some time which would be the most likely to change the avast.setup file and if changed your firewall should ask again and not simply block updates (unless you inadvertently clicked deny, etc. without realising it was for avast update).
I have also had this when whilst browsing, I suddenly couldn't access http pages, I checked my firewall permissions for ashWebSv.exe and it had permission, yet it was being blocked. The only thing to resolve it was to do what you did, delete the entry in the firewall and have the firewall ask again.
I doubt it was an issue with the firewall itself, as both my desktops and my laptop encountered the same problem at the exact same time....... as well all the others who had posted.
Whilst this does seem too much to be a coincidence when you have it happen on two computers. I doubt that all the issues with update could be the same otherwise we would be inundated at the forums with this problem. Also if it hadn't been firewall related for many then we wouldn't have been able to resolve their problem, as in the originator of this Topic.