Is there an option to turn off HTML?
I think not.
Why has Mozilla decided that this is the right thing to do?
This seems very reasonable, but removing options from dumb users also removes them from the expert user - and that's us. Reducing freedom, even freedom to crash the application, can be seen as a bad thing. And if reducing that freedom exposes the browser user to all manner of nasties, then it is even more a bad thing.