UOA SSO Fixer от pingin

Based on this extension: https://chromewebstore.google.com/detail/uoa-sso-superhero/lmafgonodcceimoijocinhjoknmdljol by Blimp, ported to Firefox.

Automatically fixes the Auckland SSO error page by removing the error code at the end of the url. Requires site data permissions for the uoa page.

It is literally just this js line:
window.location.href = window.location.href.replace('&errorCode=999', '');