I just tried that fix and you didnt have to restart Firefox. Though if you are the type of nerd who gets the nightly releases, you will have to set this to false every time you get the new build.
I would say there are very few IE fixes this easy.
Hey guys - it's not working! I confirmed on the demo page and the forum, and they are getting the same response - THE IDN FALSE WORKAROUND *DOESN'T* WORK FOR FIREFOX 1.0. Evidently this workaround only performs in 0.93 - we should see more on Mozillazine.
http://forums.mozillazine.org/viewtopic.php?t=214828
Once again - the workaround does NOT work for Firefox 1.0. Confirm on the demo page before you assume it works in your browser...