First Fire, Subsequent First Fire (SFF), and Final Protective Fire (FPF) can be confusing when you're just learning the rules. Here's an example with a 4-6-8 defending in open ground against three attacking 4-4-7s moving towards it through open ground from 5 hexes away.
First, the attackers shouldn't move as a stack since stacks draw lots of fire, and stacks don't generally stand up well against FFNAM, and FFMO. So, Squad 1, a 4-4-7 charges forward one hex. The defender First Fires at 4 (-2). Let's say the 4-4-7 survives the attack. The defending squad is marked with a First Fire counter, and 2 residual FP are left in the hex. Note that the defender can no longer fire at units that are farther away (A8.3).
Now the 4-4-7 charges forward a second hex. The defender uses Subsequent First Fire at 2 (-2) [Area Fire for SFF], and generates no result. The defending squad is marked with a Final Fire counter, and 1 residual FP is left in the hex. Note that SFF can only be used at Normal Range.
Once again, the lowly 4-4-7 charges forward. It is now two hexes away. The defending squad can't fire at the stack since it is now marked with a Final Fire counter and the attacker is not adjacent.
The 4-4-7 moves adjacent to the 4-6-8. The defender now uses Final Protective Fire at 4 (-2) [Area Fire*PBF], and rolls an 8. The 4-6-8 is pinned from the attack (A8.31). Let's say the 4-4-7 goes berserk from the NMC. The berserk squad now has 4 more MF (8 MF for being berserk less 4 MF already used). It has to charge and enter the defender's hex. The defender has to FPF (A8.31) at 3 (-2) [TPBF*Area Fire*Pinned Fire A7.81]. Let's say both squads survive the FPF. Residual fire (2) is placed in the adjacent hex, and (1) within the defender's hex. Had the defender been in a building, it's attack would have been at 3 (+2 TEM - 1 FFNAM = +1).
Squad 2, another 4-4-7, now declares Assault Move and moves into the first hex. It takes a 2 (-1) residual fire attack. The squad breaks, other defending squads could now attack it with (-2) modifiers (A8.14, A4.61). Mark it with a DM counter.
Squad 3 gets bold and survives the first residual attack, but it pins in the second residual hex. Once pinned it no longer pays the penalties of FFMO or FFNAM (A7.83). Had it gone berserk, and survived the residual fire in the 4-6-8's hex, it would force the 4-6-8 to FPF one more time, again using the result as a possible effect on the 4-6-8 (A8.31).