Even if it allowed interpreters the iOS doesn't allow just in time compiling - you can't dynamically generate and execute code. This means V8 can't be used since it doesn't have an interpreter and Trace Monkey would do no tracing.

"with Apple’s prior written consent, an Application may use embedded interpreted code" - from the dev agreement. A few games have been approved to host an interpreter. So it's just a matter of Apple deciding if they want fully featured Firefox, Chrome, Opera or not.

You left out part of it:

"Notwithstanding the foregoing, an Application may use embedded interpreted code in a limited way if such use is solely for providing minor features or functionality that are consistent with the intended and advertised purpose of the Application."

You might argue that executing JavaScript is a "minor feature" of a web browser. Then maybe Chrome would be allowed. But almost of Firefox's frontend is also written in JavaScript and tied to Mozilla's JavaScript engine. So there's no way that we (mobile Firefox) fall within the "in a limited way..." allowance.

They've approved interpreters for games for code that was written by the developers. I can't see them allowing an app that has a custom interpreter that users can use to run arbitrary code.

A 3rd-party rendering engine is a security liability that Apple cannot control, therefore they won't allow it.

Apple would never allow it.