Well, you can breathe it, although it will result in oxygen toxicity. This doesn't have anything to do with fires, and the gas doesn't have to be "more massive" than O2 to retard fires either.
Low pressure (100-200 mbar) pure oxygen behaves similarly to low partial pressure atmosphere in inert diluate (i.e. air). It's not perfect -- things still burn faster without the diluent -- but it's not as dangerous as pure O2 at sea level pressure, nor is it like the high-pressure pure O2 environment in the Apollo 1 test.
Also, N2 (and especially Ar) aren't consumed to an appreciable degree. They can be recycled in an artificial life support system (as they are, and have been, for decades).
Low pressure (100-200 mbar) pure oxygen behaves similarly to low partial pressure atmosphere in inert diluate (i.e. air). It's not perfect -- things still burn faster without the diluent -- but it's not as dangerous as pure O2 at sea level pressure, nor is it like the high-pressure pure O2 environment in the Apollo 1 test.
Also, N2 (and especially Ar) aren't consumed to an appreciable degree. They can be recycled in an artificial life support system (as they are, and have been, for decades).