Get it while it's hot (or at least before Firefox 20 comes out).
Firefox 19 was last month released in beta but already Mozilla is pushing out the finished version of the browser. The most notable new feature with Firefox 19 is a new, built-in PDF viewer to ward off the potential for security exploits due to PDF reader plugins. Speaking about the new feature last month, Bill Walker and Brendan Dahl, Mozilla Engineering Manager and Software Engineer, said that the feature was driven by a desire to push the boundaries of HTML5.
"For a number of years there have been several plugins for viewing PDF's within Firefox," they said. "Many of these plugins come with proprietary closed source code that could potentially expose users to security vulnerabilities. PDF viewing plugins also come with extra code to do many things that Firefox already does well with no proprietary code, such as drawing images and text. These problems, and the desire to push the boundaries of the HTML5 platform, led Andreas Gal and Chris Jones to start a research project they named PDF.js. The project quickly picked up steam within Mozilla Labs, where it grew into a full-fledged PDF viewer."
Firefox 19 also boasts start up performance improvements, Remote Web Console for connecting to Firefox on Android or Firefox OS, a Browser Debugger for add-on and browser developers, as well as bug fixes.