I don't think that a fire extinguisher is strong enough to serve as thrust for a person flying through space.
EVERY action causes that opposite reaction ... I remember reading a Clarke novel, ISLANDS IN THE SKY, where somebody maneuvers inside a huge zero-g space by first throwing his clothes away and later spitting his way across. I imagine a fire extinguisher would provide a lot more than that.
I just wish I wasn't sick as a dog, I've been waiting YEARS to see this movie (I watch CHILDREN OF MEN a couple times per year, it's my fave for this century thus far), it's the only flick I've felt I needed to see in a cinema in a very long while. I felt that way before AND after talking to the director and cinematographer and vfx super about the show (ICG magazine will probably have my making-of article up online pretty soon if they don't already.)