Mozilla has released first beta version of Firefox 4 today, and this version has a fresh look, latest features, and support for more HTML5 specifications.
The major changes can be observed in interface. In this version, the new-and-improved Firefox skin is only complete for the Windows release, so Mac and Linux users, you’ll have to wait a bit.

. Tabs are on top now by default. (only for windows)
OSX and Linux will be changing when the theme has been modified to support the change.
Note : If you don’t like tabs on top, you can disable them by right-clicking empty space on the toolbar chrome and unchecking Tabs on Top.

. For Windows Vista and Windows 7 the menu bar has been replaced with the firefox button.
Note : You can get your menu bar back by right clicking empty space on the toolbar chrome and making sure Menu Bar is checked.

. Inclusion of Smart Location Bar.
You can search for and switch to already open tabs in the Smart Location Bar.

. For windows, Mac and Linux, the stop and reload buttons have been merged into a single button.

. The Bookmark toolbar has been replaced by Bookmark button by default.
Note : you can switch it back if you’d like.

. Crash protection for Windows, Linux, and Mac.

. Native support for the HD HTML5 WebM video format.

. Responsive page rendering using lazy frame construction.

. The new add-ons manager, which loads in a tab of its own instead of a separate window.

. A new page loading progress indicator.