For those having a problem with the Firewall being turned off after the update, try this before you do a complete reinstall:
Open Avast Premium Security -> Press Menu in the Top Right -> Settings -> General -> Troubleshooting -> scroll down to Reset all settings and click on Reset to Default and reboot your PC
This is from Avast! Support, and it worked like a charm on both of my computers (PC). I tried the Repair and it didn't work, but this did. Hope it works for you!
Be safe!