How do I use a U2F key to log into google account in firefox on android
Well, it's not a complete answer, but Google has apparently added some behavior at this point: There's now a URL (https://g.co/sc) given when you get to needing the second factor that you can use in another device to get a one-time use code.

I was able to switch to chrome on the same phone and use that URL. Chrome then prompted me for my FIDO key, which worked fine and I was able to login to firefox.

In no way is this ideal, but at least it's a workaround.