(which is silly because you can yell fire in a theater if, you know, the theater is on fire).
Actually I don't believe you can legally. The law was created in the belief the panic from hearing fire yelled would do more harm than good as panic stricken patrons trampled over each other to get out.
I'm not sure how you notify them without striking fear into them but I'm sure the law addresses that in the details
You asked that with the sarcasm flag, but seriously if the fire alarm system doesn't sound, and only you noticed the fire, I would go to the movie theater staff and alert them so they can follow their emergency procedure. I'd imagine that procedure involves something like stopping the movie and asking patrons to exit the theater calmly and quietly, perhaps with the theater staff offering a partial or full refund or tickets to a later showing and implying (or outright stating) that the reason for the stoppage is nothing more than technical difficulties with the projection system.