See: punycode phishing / homograph attacks
They likely mean Unicode outside the ASCII range. Last I checked Firefox refuses to implement the "fix" for this type of attack because it is "culturally insensitive" and "treats English as a privileged language over other languages".
This URL in Firefox should look like "apple.com": https://xn--80ak6aa92e.com/
What do you mean by that? What characters aren’t part of the Unicode Standard? Did you get here via the IP?