Just in case anyone is wondering - this is already possible with Clang/LLVM and the work from Gregory Szorc: https://github.com/indygreg/apple-platform-rs/tree/main

The Firefox source tree has every tool available for cross-compilation from Linux → macOS, which is how Firefox is built for macOS.

The real deal-breaker is supporting the iOS app development. Which right now is only possible using XCode.