Yeah, it shows a little thing indicating you went to a punycode website. That doesn't solve anything, though, because that means you've already GONE to said website and so potentially exposed yourself. It should instead block IDN URLs from even loading and give a warning, just like warnings for redirects and such.

So yeah, this add-on works as intended/described, it's just totally useless. Really don't get why no one has done this seemingly simple thing, yet. (Nor why Mozilla refuses to actually protect their userbase by doing it themselves).

Works just as it says it does. Great alternative to setting network.IDN_show_punycode to true in about:config. Thanks, Mirclus, for making this add-one!