Firefox browser can't log in to icloud.com with error: Failed to verify your identity. Try again.

After attempting to sign in today to iCloud.com on my MacBook Pro using the latest Firefox browser (version 96.0.2 64-bit) and getting my 6 character verification code sent to both my MacBook and my iPhone there is nowhere on the Firefox browser to allow me to enter the code.

In the past on the icloud.com login page in Firefox there has always been an Apple ID Two-Factor Authentication web form with 6 input boxes to enter the verification code but now this web form doesn’t appear in Firefox.

Consequently, after a moment, an error message says ‘Failed to verify your identity. Try again.’

I am able to sign in okay using this two-factor sign in with the Safari browser and with the Chrome browser on my MacBook Pro. But not in Firefox.

Same issue on a Catalina and High Sierra MacBook Pros.

Does anyone know if this is a problem with Firefox or with Apple or even both?

Thanks.