It used to be that all scripting engines were off limits, because you could easily write a commercial app that didn't give Apple 30% of all revenue. And Firefox is pretty obviously a big scripting platform which you can do anything with.

But since then they've made so many exceptions to that policy, where the applications are important to keep people in the Apple ecosystem, that it's hardly a strict rule anymore. So while they probably could make yet another exception, they probably feel a general purpose browser such as Firefox would risk being the last exception they make.

Once you make an exception, you bet Google will want an exception too.