We've known for quite some time that Mozilla was preparing a new theme for Firefox 4. On Mozilla Links, the UI team has informed the public that the new theme will finally find its way into the browser's first beta release. According to the post, the following changes can be expected in the Windows version:
The Firefox button - a new menu button that aggregates the most frequently used browser menu options (default for Windows Vista and 7; can be disabled)
Aero Glass interface - a new skin that fits with the default theme of Windows Vista and 7 (currently only available via third-party plug-ins)
Customizable tab bar - ability to drag buttons around the tab bar
Bookmarks button - opens bookmarks menu and lets you dock it as a sidebar
Tabs on top - now enabled by default and can be changed by user