You're mistaken. It's not the rendering engine that Apple has a problem with; it would be no TOS violation to port Gecko to the iPhone. Apple doesn't allow applications that execute code, which is why it's the Javascript engine that is the source of trouble. Opera Mini is allowed because the limited JavaScript that it does support is executed server-side rather than in the app. Opera has the experience and infrastructure to make a web browser that's appealing (due to its speed) despite lacking a JavaScript engine; I think it would be challenging for Mozilla to do likewise.