I'm being fined by my landlord for using a fire extinguisher in the stairwell. What should I do?

Your an idiot. You roommate is in no way responsible for the actions of you or Matt. It's your choice to sue Matt for his share of the damages...but in the mean time, pay the bill and be very grateful that you weren't arrested for damaging your landlord property and/or evicted. For craps sake...can't the two of you take ANY responsibility for your actions.