Thank you for pointing this out. I was too eager to see it in action and - after skimming the article and obviously missing that clause - proceeded to hit the nice blue "read the JavaScript paper with pdf.js online, too" in the paragraph before the quote that contains the statement about nightlies.

My bad, mea culpa.

I won't run a nightly on my main machine to give it a try, but awesome if it made the leap to pixel perfect between FF 5 and current builds.

Apparently there's a number of bugs with Firefox's implementation of canvas and web fonts that were fixed for FF7. They rely on those fixes to get correct behavior.

(They've also found a couple WebKit bugs as well)