Don't allow it, it is not a firewall but the Network Shield, which if your firewall is doing its job the Network Shield will not have any business, it monitors common worm and virus entry points.
If you terminate the avast Web Shield can you connect ?
If so, it is more likely that the inability to connect is Outpost blocking the Web Shield, ashWebSv.exe, check the Options, Application (don't know if this is the same in OP 2008) and see if the file is blocked, if so allow it and you should be back in business.