Yes I found that build somewhere but there are also people saying how that build is not viable anymore because of PoF. I mean as you said, it is good but only when it comes to healing. That is why I am thinking of building a Gaurdian after all. Yeah Guardian can do damage during its support/heal rotation unless you need to burst heal or support in F2 or F3, about as much as the worst parsing Chrono spec (maybe a bit more). But since it can just give boons like no one's business, that is also a big draw to the Firebrand. The one downside I have seen to FB, in a PvE context, is the group needs to be stacked kind of tight for you to be at your highest effectiveness. Otherwise, you're better off as Firebrand over Tempest. From a WvW/sPvP view, I cannot comment. I just know Firebrand is wanted in those, too. And the place of Auramancer Tempest is unknown to me in PoF.
