Firewall partly breaks internet sharing over WiFi after upgrading to Yosemite

I used to share my ADSL/Ethernet internet connection over WiFi from a iMac to my iPhone/iPod/MacBook, but after upgrading to Yosemite (10.10) sharing stopped working with a very puzzling behaviour:

• connections to google services (email/dns/http) succeed;


• connections to other hosts (twitter/wikipedia/…) hang and ultimately time out;


• everything works fine if I disable the firewall from System Preferences > Security & Privacy.

The problem persists after upgrading to 10.10.2 and is not related to WiFi, which is working fine for google and other services like file sharing… any hints?