Are you guys disabling Network Shield in Avast when using together with Comodo Firewall?
I switched from Comodo to PC Tools several months (or more) back, but with both firewalls I kept the Net Shield active with no problems. In particular, the shield was noting and blocking the occasional Dcomm attack, which for some reason I couldn't get either firewall to block properly. So think of the shield as in effect a backup firewall.
[Edit, an afterthought] Come to think of it, I haven't seen any DComm warnings from the net shield since switching to PC Tools, so maybe the latter is actually catching those properly even before avast "sees" them.