tl;dr; Yes, you can use Firefox 60 in Debian/stretch with your U2F device to authenticate your Google account, but you've to use Chrome for the registration.

Thanks to Mike, Moritz and probably others there's now Firefox 60 ESR in Debian/stretch. So I took it as a chance to finally activate my for work YubiKey Nano as a U2F/2FA device for my at work Google account. Turns out it's not so simple. Basically Google told me that this browser is not support and I should install the trojan horse (Chrome) to use this feature. So I gave in, installed Chrome, logged in to my Google account and added the Yubikey as the default 2FA device. Then I quit Chrome, went back to Firefox and logged in again to my Google account. Bäm it works! The Yubikey blinks, I can touch it and I'm logged in.

Just in case: you probably want to install "u2f-host" to have "libu2f-host0" available which ships all the udev rules to detect common U2F devices correctly.