Besides the native PDF viewer, most new features are for developers
Mozilla is getting ready to release Firefox 19 to the stable channel. There's not much to get excited about in this release, but that's almost always the case ever since a new Firefox comes out every six weeks.The most touted feature is the native PDF viewer, which is finally ready for the prime time. It's been a couple of years in the making, PDF.js is stable and fast enough to be a part of Firefox.
What this means is that you'll be able to view most PDF files online in the browser without having to download them to your computer or to install any plugin.
Otherwise, the changes and new features in Firefox 19 are either under the hood or interesting only to developers.
Firefox 19 should load faster thanks to some work in the area, but you should only notice this if Firefox was loading particularly slow before.
Mozilla focused on debugging in Firefox 19, it seems, the built-in debugger now supports pausing on exceptions and hiding non-enumerable properties.
There is also a new Browser Debugger, which should be of interest to extension and browser developers. It's not enabled by default, you'll need to switch on "devtools.chrome.enabled" in about:config for it to work.
The same goes for the new Remote Web Console for the Android browser. It's possible to debug mobile pages running on actual devices now, but you'll have to enable the feature first, set "devtools.debugger.remote-enabled" to true.
Firefox 19 also comes with a few additions to web technologies support, the @page CSS property is now supported, as well as new viewport units, vh, vw, vmin and vmax, standing in for viewport height, width, minimum and maximum. Also new is support for the full-width property in text-transform.