B767-300 Captain reported a cargo fire warning while being de-iced on the ramp with the APU running.

Narrative

This was not technically an actual flight as the aircraft never moved from its parking position at Gate XX. The FO and I were on the aircraft with the APU running; and the aircraft was being de-iced/anti-iced while a main deck fire warning sounded. This all happened between XA:45 AM and XB:00 AM LT. The QRH NNC (Non-Normal Checklist) for Main Cargo Fire was accomplished followed by the Evacuation Non-Normal Checklist. Also; I radioed the de-ice crew to stop the application of de-ice/anti-ice fluids. The FO was reading the QRH; and actioning the Main Cargo Fire checklist items. I made sure we had the right checklist. Before starting the Evacuation Checklist; I called the ZZZ Ground Controller to send the fire trucks. During the execution of the evacuation checklist the FO inadvertently turned the right engine fire handle and discharged engine fire bottle #2.I peeked in the main deck cargo by opening the main cargo deck door to see if a fire was burning; and if there was how far away it might be from the flight deck. I did not see any fire; but could smell smoke. I knew we had some time; so I then opened the entry door to signal for crew stairs. The FO and myself put on the emergency egress harnesses in case the crew stairs were not up in time; and a fire were to break out. Both of us were ready to jump immediately if needed; and I had my window open if somehow an escape from a window would become necessary. The FO got off followed by myself without sustaining any injuries. We were greeted by fire trucks and fire personnel. I had to make a written statement of what happened; and I was asked what type of cargo was on board. I didn't recall seeing anything on the NOTOC (Notification to Captain) that was hazardous; only cut flowers. I told one of the firemen where to find the NOTOC. The firemen must have taken our copy with them after their careful inspection of the aircraft. The FO told me he remembered seeing one of them take what he thought was the NOTOC.Once the airplane was clear the FO and myself returned to the flight deck; and the logbook page was filled out while having a brief chat with maintenance reps. There was another individual who showed up; and asked about the situation because the TSOC (Transportation Security Operations Center) was notified. Transportation then took us back to the company crew facility.