It’s actually a configuration change that disables the IDN functionality in the browser. If you go to this site, you can follow the instructions and download a patch that will make the change for you - or you can type about:config into the address field and hit Enter. Scroll down to network.enableIDN and right click on it.

In Firefox, select Toggle, close all Firefox windows, and restart it; in Mozilla, select Modify and change true to false. Close all Mozilla windows and restart it. After you’ve made the change, check the about:config page again to make sure that the change has taken effect. The Mozilla team is working on an update that will fix the hole and restore the functionality - you’ll have to wait for it to be released.