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Smart Location Bar(with restricted access)
Web standards improvements in the Gecko layout engine
Support for CSS 2.1 and CSS 3 properties
Tab-switching shortcut that shows previews of the tab you’re switching to
Support for new web technologies such as the video and audio elements,
W3C Geolocation API,
JavaScript query selectors,
Web worker threads, SVG transforms and offline applications
Mozilla developers quoted that :Users who download the beta should not expect their add-ons to work properly. you can provide feedback to Mozilla you can file a bug too.
Known issues in firefox 3.1 : Copied From Mozilla 3.1 release note
This list covers some of the known problems with Firefox 3.1 Beta 1. Please read this before reporting any new bugs.
All Systems
Certain client certificates will fail to install when downloaded from a CA (see bug 453227)
The “Feed” tab is not present in the page info window (see bug 458579)
Preferences set using the page info window do not work properly (see bug 459157)
Microsoft Windows
In some cases, the migration dialog will not launch after successfully installing Firefox for the first time (see bug 459082)
Mac OS X
On PPC Macs, viewing a Ogg video, displayed in a
