They would certainly prohibit them "replacing" Chrome with FireFox, but the suggestion that they would stop an OEM shipping an alternative browser on a phone is such a ridiculous piece of FUD it's laughable.
This whole thread is so full of FUD. Google is not blocking firefox. There's a whole bunch of green-named people who have decided to sign up for an account and pretend that google is going to block firefox. There is no evidence that google is even considering this, and no valid arguments as to why google might want to do it. it's bullshit.