If you want to share the same code base you need to grantee the same functionality. The easiest and most predictable way is to run this through an intermediate and then render that intermediate. This is why compilers use intermediate representations like bytecode, you don't want to write a compiler that builds only x86 and then another when you need to build ARM. You don't want to write a rendering engine with different implementations for each widget kit.
Well, the XPCOMs represent the bytecode in your opinion. One still can't get an XPCOM for FF to work on FFoS, I meant the simplest one without making huge changes to the code base. Yes, one UI engine is needed and so is a (cross-)compiler in your case OR alteration of the code base for FFoS of FF to ensure portability. I guess it would be the FFoS code base that might need to change. BUt that could be tricky since it leverages the Andriod's core - a component-based linux OS.
If you want to share the same code base you need to grantee the same functionality. The easiest and most predictable way is to run this through an intermediate and then render that intermediate. This is why compilers use intermediate representations like bytecode, you don't want to write a compiler that builds only x86 and then another when you need to build ARM. You don't want to write a rendering engine with different implementations for each widget kit.