Comment by akrakesh

And, adding this to userChrome.css removes all rounded corners

    :root {
     --border-radius-small: 0 !important;
     --border-radius-medium: 0 !important;
     --border-radius-large: 0 !important;
     --border-radius-xlarge: 0 !important;
     --border-radius-circle: 0 !important;
    }