An actual firefox for iOS is disallowed from by the app-store rules.
It's still built on WebView.
You can run a browser in an interpreter but anything modern would be very unpleasant to use that way.
> apple won’t let you publish an app that can
Agreed, but we're talking about whether you can technically make such an app, say to run in dev mode on your own personal device.
https://siguza.github.io/psychicpaper/ is a recent writeup that includes more details on (escaping) the limitations.
Safari is a special case. It is allowed to use JIT and writable executable pages, but this is not allowed for third party apps.
It is definitely technically possible on iOS, but a significant challenge for third party developers unless using a jailbroken device.