The release notes have a very clear overview of the new features, so I figured I might as well just copy that to make sure I get everything:
- Available in more than 70 languages. (Get your local version!)
- Support for the HTML5
<audio>elements including native support for Ogg Theora encoded video and Vorbis encoded audio. (Try it here!)
- Improved tools for controlling your private data, including a Private Browsing Mode.
- The ability to share your location with websites using Location Aware Browsing. (Try it here!)
- Support for native JSON, and web worker threads.
- Improvements to the Gecko layout engine, including speculative parsing for faster content rendering.
<canvas>text, ICC profiles, and SVG transforms.
You can obviously update your installatiomn of Firefox to this latest version via the built-in updater, or your Linux distribution's repositories will handle the matter. If you currently do not have Firefox installed, you can grab the browser from the download page for your specific platform/locale combination.