So, one stable release without something like a two-factor authentication method. Since the addon will break and no official support will be released on time, there's no other choice but to use Chrome/Chromium to use Yubikeys.
Or, you know, just use the ESR release for a few weeks.
So does that mean Mozilla don't use 2FA?
2FA is actually globally enforced for all internal Mozilla accounts, IIRC---it's just not done over U2F. I'm not sure whether the details are public so I can't elaborate further.