In the early part of the war, with the rapid expansion of the fire services, initially there were not enough uniforms for everyone. Post Office uniforms were issued to some members of the AFS/NFS on a temporary basis. This may account for the different headgear. Women were also recruited into the fire services to man mobile canteens which may explain why Mary is not in full uniform. Obviously a hat and coat would be essential, but in a canteen perhaps, ordinary civilian dress was acceptable at that time.
Women also served as drivers and communications personnel as well as fire watchers.