Rule 2.5.6 of the Apple App Store Review Guidelines forbids the usage of third-party browser engines:

2.5.6 Apps that browse the web must use the appropriate WebKit framework and WebKit JavaScript

Notably, this means that Firefox can’t be shipped with Gecko on iOS. It also means that Firefox can’t be distributed in the App Store on other Apple platforms like macOS. The App Store policy should be updated to allow third-party browser engines for web browsers.