For around a month so far, on what the developers are calling pdf.js, they've been developing the piece of code quite openly over at GitHub with the purpose of creating something that renders the files fast. You can try out the still work-in-progress reader yourself; the developers claim that they were not planning on releasing details for awhile, but decided to do so on the basis of the amount of interest shown in their work.
While it will be a few months before technical users can start to use the PDF feature, this could be seen by some as a fundamental change in what Firefox can do and features the casual user might come to expect from internet browsers as a whole. By having a native PDF renderer, internet surfers do not have to worry about drops in usability and the threat of security exploits. Mozilla has said themselves that the pdf.js uses only safe web languages and doesn't contain code which could be exploited by others.