From your home directory, enter the .mozilla/firefox/*.default folder; then with vim open compreg.dat, and search for the string: "idn-service;1" (use the / function). Change the 1 to 0 in both the strings you find. Now, restart Firefox.

The url will still appear spoofed at the bottom-left corner of the browser, but if you click on the proof-of-concept link it won't work.