`network.enableIDN` apparently. Interesting reading (2005) here:

https://bugzilla.mozilla.org/show_bug.cgi?id=279099

-- EDIT:

It doesn't 'disable' as such, but renders the 'punycode' in full, so a fake 'http://www.miсrоsоft.com' is rendered as instead 'http://www.xn--mirsft-yqfbx.com'.

For comparison, here's fake on top of real (not in monospace since it destroys the illusion):

http://www.miсrоsоft.com

http://www.microsoft.com

Thanks! Setting "network.enableIDN" seems not to work, but this setting works: network.standard-url.encode-utf8 [0]

[0] http://kb.mozillazine.org/Network.standard-url.encode-utf8

EDIT: doesn't work either. And the same for network.standard-url.escape-utf8. :(