It did not rely on a Chrome CVE, it just included a Chrome exploit.

What they found was a server that served up the correct exploit for the platform making the request, and were able to coax a bunch of iOS and Chrome exploits out of it. This doesn't mean that the attackers didn't have other browser exploits or that Firefox exploits couldn't have been dropped into the same framework.

In fact, if you start trolling around in Tor, you're likely to find a ton of Firefox exploits because Tor Browser is obviously based on Firefox.