First thing I really want to do is change the stem from a 60mm to a 90mm. I just feel like I am hanging onto the handlebars and pulling myself onto the bike all the time. Whereas on the Mach 4 I am usually pushing myself away from the handlebars with my palms.
I'd agree that a 60mm stem would feel too short on an AM bike like the Firebird. But unless you're feeling cramped behind the bars i'd be inclined to leave the stem around 75mm, as jumping from 60mm to 90mm would have a significant effect on the handling incl. slower turns. (
correct me if i'm wrong). Given that a badass like the Firebird should be more at home on the downhills, I wouldn't try to make it feel more like your Mach 4. Pure XC bikes are going to have longer stems for climbing and less twitchy steering, which makes them better suited to their type of trails.
My XC bike has a 120mm stem, and my AM bike 75mm. When I first rode the AM bike I had the same reaction (and experience) as yours - too short! But I gave it a chance and have gotten used to it. I just had to get used to the idea that they were completely different bikes with different purpose, and the shorter 75mm suits the purpose. Maybe give it a few more rides and see if it grows on you?
Of course this is just my opinion and definitely do whatever makes your ride feel better for you