Would be nice. Even with the experimental settings turned on in about:config, I could only read input from my Yubikey, I couldn't add one. I had to install Chromium just so I could add my Yubikey to my Google account.
This is mentioned as a side note in the first comment of the Firefox issue: they were explicitly whitelisting the “Sign” operation so that registration didn’t work.
I think this is because of Google - they only allow registration via Chrome (to the best of my knowledge). But then you should be able to use your key from Firefox.