> My understanding is that WebKit is the only rendering engine permitted on iOS as a security precaution, ostensibly taking Apple at face value and in good faith.

Look at mac. Do we have a problem with firefox on mac? Both mac and iOS share the same OS kernel. Using security as an excuse is laughable.

I once read on twitter from a mozilla dev saying they were even willing to build a browser engine without JIT just to satisfy the security requirement. But eventually they went with webkit to build Firefox for iOS.