I (think) I understand what a firewall is (at least, in a single engine aircraft where the engine is at the front of the fuselage), but what does it mean "to firewall" an aircraft's engines, as described in this incident report: is it just applying full thrust? What is the connection with the physical firewall?
"To firewall" is a phrase meaning to go to full power. Most aircraft throttle controls provide full power when moved to their furthest forward position - the direction towards the firewall separating the nose mounted engine from the cockpit in aircraft in the past. The phrase is still used, just as we "dial" a telephone even though the telephone dial is no longer used, either. A similar one for automobile driving is "pedal to the metal".