This is currently happening to me across multiple platforms. Almost exactly as the original post described. New installation(s) of Firefox, I attempted to login to my account to enable Sync etc. I had originally forgotten my password, but finally remembered it, got past the sign in dialogue to the ‘verification code’ dialogue, but no matter how many times I try (on laptop, desktop, mobile, within Firefox, within Chrome etc) I constantly get looped back to the sign-in page.
I should note, the authentication obviously correctly identifies the account because both my primary email and backup email accounts get the same two-step verification codes sent to them. I have about 20 of these now, but none seem to work. This is pretty frustrating as there’s no feedback whatsoever that something has failed or isn’t working.