The PDF file format is incredibly popular, and it's almost always used when sending documents to someone over the internet. Mozilla recognizes the PDF's ubiquity and has released Firefox 19 today, complete with a built-in PDF viewer. This allows users to skip the browser plug-ins altogether when opening up PDFs in Firefox.
The built-in PDF viewer in Firefox is fairly interactive as well, and can do most of the basic stuff that external PDF readers are capable of, such as changing the size view, searching the document for certain words, viewing page thumbnails, printing PDF documents, saving the PDF to your computer, and even bookmarking it in Firefox for viewing later on.
Firefox 19 comes more than a month after Firefox 18, and this newest update also includes remote debugging, a handful of bug fixes, CSS improvements, as well as your general performance enhancements that are always welcomed. The PDF viewer is built on HTML5, so it's fast and secure. However, there's no pinch-to-zoom support on the Mac, which is a slight bummer. Hopefully they'll add it in the future.
Of course, Google Chrome users have long been enjoying a built-in PDF viewer, so it actually took Mozilla a long time until they finally introduced the feature. Nonetheless, avid Firefox users (which seem to be dwindling in numbers lately), finally get to throw out those annoying PDF reader plug-ins for a handy default solution.