Apologies for a bunch of links headed your way, but I think I know what issue you’re having here. Keys only enrolled as U2F are not supported for Firefox, so you’ll need to make sure the key is enrolled as a WebAuthn device and make sure you’re selecting it in the prompt.
You’ll need to do the following:
- Update your Security Key in Chrome as shown here: https://guide.duo.com/security-keys#existing-u2f-users
- If it is not your default device, in Firefox you will need to select your key in the device dropdown as shown in the second secreenshot here:https://guide.duo.com/security-keys#security-key-auth
It sounds like you’ve probably already done the first bit, but it’s good to check. You can set your default device in the Self-Service Portal (“My Settings & Devices”) as documented here: https://guide.duo.com/manage-devices.