in my opinion, mini humbuckers are beautiful things and I would never rout out a firebird to put fullsized humbuckers.
having said that, the current production Gibson mini humbuckers are not well thought of, and so would be worth changing.
i've tried a few brands:
Kent Armstrong ones are not good (though I like one or two of their (other) designs)
Lollar ones are top quality, probably the best I've tried
Gemini Pickups know their mini-humbuckers very well and I have a couple which I like
DiMarzio have just released some Paul Gilbert signature mini humbuckers which would be worth looking at, though I haven't tried them.
A couple of points to note, which you may already know. Mini-humbuckers and firebird pickups have the same form factor, but different construction internally. Both are bright, but the firebird ones are brighter since they use blade pole pieces I believe. I'm not the biggest mini-HB expert but I am a fan.
A configuration worth looking into, actually one of my favourites, is to have a mini-HB in the neck and a full sized HB in the bridge. Adrian Smith (Iron Maiden) for example uses this. The advantage is that you have the grunt of the fullsized HB in the bridge, but in the neck you have all the cut and clarity of the mini-HB.
A few people on here have fitted Rebel Yell pickups in their Firebirds and like the results.