As you can see from my signature I also use Outpost Firewall Pro, but I don't use threatfire (I have never felt the need) and I have no problems.
The new avast 5.1 has a greater Behaviour Shield, though it is currently running in passive mode gathering data, what it finds is used to fine tune this shield to prevent it being to sensitive and have false positives and not too weak not to catch suspicious behaviour. To me that possibly raises the likelihood of conflict between threatfire and avast's behaviour shield.
However, since you removed threatfire first, that potential for conflict shouldn't exist, so I don't know why it would effect Outpost in this way.