Idn_navbar is a project mainly written in JavaScript, it's free.
A Firefox Add On. The IDN (International Domain Name) Navbar add-on provides address bar support for all IDN's, including '.com', '.net' and '.eu'. An 'IDN' address bar
Add-ons for Firefox Extensions IDN Navbar
IDN Navbar 0.0.1
The IDN (International Domain Name) Navbar add-on provides address bar support for all IDN's, including '.com', '.net' and '.eu'. An 'IDN' address bar icon will be visible when this add-on has decoded a Punycode URL to it's IDN.
https://addons.mozilla.org/en-US/firefox/addon/idn-navbar/