Sort of. It's true that Firefox doesn't enable U2F by default (yet), but that's actually unrelated to the reasons that it doesn't work on Google domains. The real reason that Google's U2F only works in Chrome is that they rely on non-standard implementation details: https://twitter.com/ManishEarth/status/931534674224062464
For contrast, you can use U2F on Fastmail and Github in Firefox, because they don't rely on non-standard behavior.
The point is that U2F on Google logins only works in Chrome. Firefox supports U2F, but Google's implementation only works with Chrome.