However, Tjin: "Insurance covers fires, not corrosion on electronics" isn't always the case... at least regarding MY policy (which I don't think is atypical). Coverage is present for incidental damage from fire-fighting efforts, too. YMMV
I do not feel like having arguments with the insurance about all the electronics that fail a few months later. Sure they will pay for the initial damage, but powder gets into everything and destroys them with a delay.
As for the effectiveness of fire extinguisher on car fires. Unless it's a small fire, just wait until the fire engine arrives,