Mozilla introduces several changes to the Firefox 4 beta compared to previous versions, not the least of which tabs are now on top by default (Windows only). And on Vista and Windows 7, the menu bar has been replaced with the Firefox button. All in all, the UI changes add up a to browser that looks a lot like Chrome.
More than just a face lift, Firefox 4 includes full WebGL support, though it comes disabled by default. Mozilla beefed up HTML5 compatibility, including native support for the WebM video format.
Give the release notes a glance
here, and if you're feeling frisky, download the beta release
here.