The stock is down by nearly a third since the first fire was reported on October 2. In that case, the car hit a large metal object on a state highway near Seattle.
In the second case, in Mexico, a Model S burned after a high-speed crash. Last week, a Model S caught fire near Smyrna, Tennessee, after its driver struck a trailer hitch.
Musk, who described himself as "somewhat of a perfectionist", said Tesla would recall the Model S immediately if it thought the car had a safety problem.
Ultimately, that decision might not be left to Tesla. The US Government could decide to recommend a recall of the Model S, and could take the company to court if it refuses.
Tesla factory staff injured
Tesla Motors says three employees have been injured after an aluminum-casting press
failed at its factory near San Francisco.
It said there was a failure in a low-pressure aluminium-casting press, and the three workers were hurt by hot metal.
Fire trucks were seen outside the factory but a fire employee said there was no fire at the plant.
Tesla has made headlines over fires involving its Model S electric car.
-AP