That video looks to me like a nasty example of ‘hidden ridge’, a known illusion where terrain in the foreground has a similar appearance to that in the background and ‘disappears’ until it starts to bloom rapidly at short range. The usual defence of GPWS is not so effective if warnings are experienced frequently while maintaining a safe flightpath, as is often the case at low level in mountainous terrain: a certain degree of desensitisation is likely. Other defences are to fly along the grain of terrain (not always possible) and to watch carefully for changing sight line rates of approaching terrain. I have no fire fighting experience myself but I’d imagine that the pilots are well aware...