Oxygen isn't a source of energy. It's an oxidizer. Oxygen supplied to the manifold doesn't get released into the cabin except where the pin is pulled for a specific mask. Where the pin is pulled, the mask is in use, and where the mask is in use, the oxygen is used in respiration by the user...not contributing to a fire. Additionally, to increase fire intensity, the oxygen would have to be applied at the fire site.
As stated before, I've worked for several operators which had policies and procedures for deploying masks any time environmental contamination was a problem (eg, smoke) or loss of atmosphere occured (depressurization). That's one of the reasons there's a provision to manually drop the masks; to give the crew the option when it's not released by the altitude switch. A cabin altitude event is not the only reason to deploy the masks.
Without the mask, one is breathing cabin air anyway. With the mask, one is breathing cabin air with a higher percentage of oxygen, and that's the point.