There is a patch for the ISDN flaw in Firefox, which in the newer versions is disabled
in config by default, because of phishing risks, download the patch here: https://addons.mozilla.org/messages/307259.html and install opening with firefox.exe,
It is not only a very old patch that has been implemented in newer versions of FF, but it is also not a patch for ISDN but for IDN
IDN:
Internationalized Domain Name Also known as multilingual domain names. It gives the possibility to use more characters in domains names, besides letters and numbers, and specifically characters available in other alphabets, like accents. A standard is being developed by the IETF.