Today they need to ensure it works on the iPhone, and if they are already testing two browsers, or ensuring capability across two browsers, that means Firefox is likely coming along for the ride.
With this move I’m preparing for a future where developers simply put up a message, “best viewed in Chrome”, or they simply do a user agent check and don’t even load the page if it’s not Chrome. Not because the page wouldn’t work in Safari or Firefox, but rather because they couldn’t be bothered to test.
I don’t see this as refusing progress, I see it as defense against a monarchy on the Internet, where Google holds all the cards.