Mozilla published the first beta version of Firefox 4, you can give it a try over here. One of the major changes in this new release is the more modern user interface, by default the tabs are now on top like in Google's Chrome browser, and there's a new Firefox button that combines all menu items into a single button to reduce clutter.
Here's an overview of the new features in Firefox 4 Beta:
New Add-Ons Manager
An easier way to manage your add-ons and discover new options for personalizing your browsing.
WebM and HD Video
As pioneers of HTML5 video standards, Firefox also supports the WebM format so you can watch open HD quality video.
Protecting Your Privacy
Firefox puts your privacy first, fixing flaws in some web standards that allow bad guys to snoop around and expose your browser history.
Developers will be able to build real-time online interactions like gaming and chatting.
Stylin’ Pages with CSS3
With support for new CSS3 features, web designers can make their pages both look prettier and display faster.
A new HTML5 parser and full support for web video, audio, drag & drop, and file handling means Firefox 4 is ready to run the best web apps of both today and tomorrow.
With this experimental analysis tool for modern sites, Firefox allows you to peek under the hood of dynamic web pages.
JetPack SDK (Making Add-On Development Easier)
Add-Ons can be installed without restarting the browser, and can be developed more easily using the new JetPack SDK and js-ctypes.
Developers will be able to store application data locally, making cloud applications like Gmail or GDocs work more quickly and reliably, even when not connected to a network.
Firefox provides uninterrupted browsing for Windows, Linux, and now Mac when there is a crash in the Adobe Flash, Apple Quicktime or Microsoft Silverlight plugins. If one of these commonly-used plugins crashes or freezes, it won’t affect the rest of Firefox. Instead, you can simply reload the page to restart the plugin and try again. Learn more about plugins.