User agent gives the browser version and platform version. Two macs with the same OS version and the latest version of Chrome will have the same user agent.
That's the point. With this feature, two computers with the latest version of Firefox would have the same font list.
I agree that implementing this first in Tor is probably not a good idea, but if Firefox were to do it first, then I don't see the problem. "They're a Firefox user" isn't nearly as specific information.