Mozilla have released the Beta 1 version of Firefox 4. The specification looks great and includes support for HTML5 video and WebM format HD video, further support for CSS 3, crash protection, a new Add-Ons manager, a new SDK called JetPack, an Indexed DB for developers to use to store data locally and a brand new User Interface.
I have been using it for a few hours and it seems to be a great upgrade. The new UI is only available for Windows users at the moment, with Mac and Linux versions to follow soon.
You can take a look at the new features at http://www.mozilla.com/en-US/firefox/beta/features/ and get a download of the Beta 1 version at http://www.mozilla.com/en-US/firefox/beta/