How do I use a U2F key to log into google account in firefox on android

My u2f key is already setup and working as the second factor on my google account, and I've used it many times, including using it with this tablet via usb. I now want to log into my google account in the firefox browser on my galaxy tab a. When I try, I only get the option to enter a backup code. I checked about:config and security.webauth.u2f is set to true. Security.webauth.webauthn is true. Security.webauth.webauthn_enable_usbtoken is true. What am I missing here?