You are in danger of being percieved as missing the point entirely
The issue is fire fighting by oxygen starvation
not,
people fighting by oxygen starvation
wild goose - The percentage doesn't matter, the air is still far less dense so there will be less oxygen to feed the fire. If you are in a near vacuum and have 100 "air" molecules and 21 are oxygen, you probably don't have enough to feed combustion, though your ratio is still the same.