Windows users will notice a big change in interface design, which, among other things, has tabs located along the top by default. There's also a new Add-Ons Manager, which affords more space to managing add-ons, themes and plugins. Firefox 4 Beta 1 brings support for HTML5 and WebM, improvements to privacy, crash protection, and a boost in performance when it comes to start-up and page-load times.
Features expected in the future include the ability to sync your settings, passwords, bookmarks, history, open tabs and other customizations across multiple devices; better privacy controls; yet more improvements to performance; and the same themes for OS X and Linux.
Mozilla's Mike Beltzner warns that not everything in this release will make the final cut but, as always, the best way to ensure your favorite features are included is to get in on the Beta and provide feedback to that effect. FF4B1 even includes a Feedback add-on to make it easier to send in your likes and dislikes.
Download Firefox 4 Beta 1 for Linux, Windows or OSX here and let us know what you think are the biggest and most important improvements.