IDN's are great for linguistic diversity but can be used for phishing or spoofing, so with IDND, suspicious IDN's cause an alert box to appear, warning you to take care. (This is in no way sufficient protection against all kinds of spoofing, but does add to your browser's defences.)
In addition, you can right click the URL address in the Firefox navigation bar and select IDN to get more information on the domain name (whether/where the domain contains international characters).
Finally, some users may wish to right click the navigation bar itself, choose "customise..." and then drag the IDND icon to beside the URL address. This icon changes green/blue/red depending on the nature of the domain name, it tells you whether you are visiting a Traditional Domain Name (green TDN) or an International Domain Name (UN-blue IDN) or an IDN suspicious domain (red).