Although you could keep avast 6 for now (and it should work fine too), the next time you try avast 7 you should delete the firewall rules related to web browsers and to avast; that is, if the same problem appears again. After the installation and reboot, the firewall (software and/or hardware) may not recognize correctly the new versions of avastsvc.exe, avastui.exe avast.setup. So by deleting such rules and a new reboot, the firewall should recognize them again (and maybe even ask for permissions).