Defense would have to show that the "improper installation" affected operation in such a way as to cause or contribute to the fire moreso than faulty design.
Actually the plaintiff would have to show a design error, the respondent has more than one option and in fact can pursue several differing and what might appear to most folks conflicting lines of defense.
I also am not a lawyer but have seen several very complicated civil cases work their way through the federal court system. Ending in total destruction of the company that was the plaintiff. I don't bet on which way a case will proceed in court.
Anyway the OP is in Canada.