Have never used Outpost firewall so cannot offer much help.
Probably should change the topic of your post to something like, avast! Home compatible with Outpost Firewall? Which is what your post is really about, and would catch the eyes/interest of avast! users that are using Outpost.
Per my sig below, I'm using avast! with Online Armor Premium and MBAM (resident) without any issues/problems. Also using avast! with SAS Pro (resident) on another system.
Good luck!