Firing order is cast into the intake manifold... but there's a simple way to remember it. 1 5, 3 6, 2 4 - "too young, too old, just right." Each pair of numbers adds up to something divisible by 3 and the middle is bigger - 1+5=6, 3+6=9, 2+4=6.
It might not be all that simple but I bet you won't be able to forget it now :gee: