For the past few weeks I’ve been using XULRunner to create an easy (no need to constantly reinstall) environment to develop extensions under. But I haven’t really used it to run anything useful up until now, tonight I just packaged ChatZilla and I must say I’m really liking the abiltiy to run my favorite extensions without a browser open. ChatZilla also seems to be quite a bit more stable under XULRunner (under Firefox, CPU usage would skyrocket).
Once XULRunner is officially released, I think it’ll become a great platform for creating various applications (XUL and XPCOM can be used greatly reducing development time). For the time being, if you want to run a great IRC client without having to run Firefox or Mozilla in the background, give XULRunner+ChatZilla a try.