It will still dmg true, but not enough that you should treat it the same as a P4. But you should never be trying to fight around an enemy tank without some supporting AT.

A few questions, How spaced were the AT guns? How close were the your tanks? Did you have any more infantry ahead of your line to spot the rush (besides sappers)?

As soon as you see an enemy tank, let the firefly take the first shot, then pull it back, Allied AT tanks should have the reverse ready at all times. Spin around the at guns to cover as much area and focusing around your tanks if a rush is starting, you might lose yours but they will lose all of theirs too. In an all out engagement Cromwell’s and other med tanks become dispensable, use it to block the panthers movement and by some time.