A small fire extinguisher is only good for about 15 seconds.

A B/C extinguisher is useful for flammable liquids and organic solvents (the B part), and electrical fires (the C part).

I would stay in contact with the police and fire departments and ask if they have made any discoveries in relation to this incident.

Could this have been an irate previous tenant?

If your landlord doesn't want to install a lock on the door, pin him down and insist on knowing WHY he doesn't want one (maybe as a group). I'm not familiar with lawsuit laws in Canada, but if he takes no steps to prevent a repeat episode, he may be setting himself up for a major personal loss.

Are there any other issues going on with this place? If so, could they be connected?

Sue