
There is almost nothing to do beyond removing a competing second leader and deadwood, and that work has to happen while the tree is young. The trap worth telling clients: fir does not resprout from bare old wood, so a branch cut back off a roof or wall never fills in again — you cannot "cut a fir back," and limbing up the low skirt is permanent.
Dieback starts at the top and the leader and works downward, which is the signature of a root problem and the opposite of the bottom-up interior browning you see from foliar disease on spruce. Interior needle shed and some color fade are normal; a limp or dead leader with needles that pull off green means check the root flare and the drainage, not the foliage.