See:

• https://support.mozilla.org/en-US/kb/reset-your-firefox-account-password-recovery-keys

Using Translator

I know how. but

I couldn't save it.

I can't log in because I don't have a recovery code.

I'm logging in with a different account now.

When 2FA is enabled then you still need the 6 digit 2FA code or use a recovery code. The recovery key is meant to be used instead of the password.

If you lost both the recovery key and possible recovery codes and you can't access the setting page via another connected device then your only option is to create a new Firefox Account using a different email address.

• https://support.mozilla.org/en-US/kb/what-if-im-locked-out-two-step-authentication