Avast main GUI -> Web Shield -> expert settings -> "...well known browser only".
Save and reboot (not necessarily needed, but best for troubleshooting).
This may or may not be your problem. I am only guessing, and there are many other possible conflicts/solutions.
About the default firewall in Windows Seven, it shouldn't need any specific rule, unless (maybe) if you are using advance firewall settings.
Change the Avast setting, delete the firewall rule (recommended anyway), reboot, test, and report back.