OK so I just reread what it said. Never really says which tank hit with the rounds. At first I thought the tiger hit the firefly...now it looks like the firefly hit the tiger. 
Read it one more time.
There was no Firefly involved in this encounter.
3 Shermans, 1 Tiger, 1 17 pounder anti-tank gun. The Shermans engaged the Tiger. Tiger Kills one Sherman. 17 pounder set up 3,000 yards away, engages Tiger. Tiger turns face on (90 degree turn) to anti-tank gun. Facing the threat with it's strongest armor. Tiger is hit 2-4 times, ineffectively due to the great range. Shermans move back up to within 500 yards of Tigers flank and engage. Tiger withdraws.
What I gat out of this is:
1. Tiger kills one Sherman from 500(?) yards.
2. 17 pounder anti-tank gun is ineffective from 2,400 yards against the front armor of a Tiger.
3. A (regular) Sherman's gun is ineffective against, or missed, a Tiger's flank from 500 yards.
wrongway