While it's no longer available as a stand-alone (except possibly from sites that specialize in older versions), I've never had any problems with the PC Tools Firewall Plus (free version) and avast. And since, as craigb pointed out, firewall updates are few and far between and rarely critical, it's no big deal that PC Tools (I think now one more acquisition laid to rest by Symantec) no longer supports it.
One of its features I like (and in any firewall including it that I've used) is that it'll flag application rules for apps no longer on your system and let you conveniently delete those rules. The only such rule I leave in place anyway is the one for avast.setup, so it doesn't have to ask me for permission each time there's an auto-update check ... apparently avast.setup doesn't change enough between its successive creations to trigger a "file has changed" warning.
