Hi nin28
I ran over the same problems, I´m running Comodo 3.0.22.349 on Vista x64 SP1 and Filezilla FTP Server 0.9.25.
After solving my problem running the Filezilla Server as a Service with activated UAC (
Directlink to FZ-Forum with my post) and installing the Comodo Firewall I searched help in this forum - i found out that using a
global rule with:
Allow TCP - In/out
"source adress", "destination adress" and "source port" = "any"
"destination port" = "port range"
start port=20 end port = 21(and applying the rule using the "apply" button) fixed my problem smoothly

Hope this helps you too.
p.s.: ofcourse the filezilla server exe also needs full access in "application rules"
p.s.2: and ofcause the new global rule has to stand atop of the global block all-rule