Since installing avast! a few days ago, I am unable to surf to any ftp sites. Is avast blocking them? If so, how can I allow specific sites?
Avast is not blocking ftp in any way. Avast is a scanner, not a firewall.
And if avast blocking something it showing messages that page/access was blocking.
Have you some more program installed that monitoring your computer,
as firewall or anti-spyware or one more antivirus or some other scanner type?
Do you using some router?
Thanks, your right. 
Should have thought of that.