From the For Devs Eyes Only (ok not really, but c'mon!) dept.:
New features and changes in this milestone that require feedback include:

* Web standards improvements in the Gecko layout engine
* Added support for CSS 2.1 and CSS 3 properties
* A new tab-switching shortcut that shows previews of the tab you’re switching to
* Improved control over the Smart Location Bar using special characters to restrict your search
* Support for new web technologies such as the <video> and <audio> elements, the W3C Geolocation API, JavaScript query selectors, web worker threads, SVG transforms and offline applications.

mozilla.org