The article mentions that pdf.js uses some new Firefox features. It should work correctly in FF7 (nightly) and not FF5 (stable).

"We said above that pdf.js renders the Tracemonkey paper perfectly … if you’re running a Firefox nightly. On a Windows 7 machine where Firefox can use Direct2D and DirectWrite. If you ignore what appears to be a bug in DirectWrite’s font hinting. The paper is rendered less well on other platforms and in older FIrefoxen, and even worse in other browsers"

Yeah, it seems to be a Linux thing; Andreas was explaining that the font-rendering is highly platform-dependent.