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

Regarding tabs on top, many users have been quick to call Mozilla copycats. Opera, as well as Google Chrome, have both used tabs on top for quite some time. However, to set the record straight, Mozilla wants to assure the public that it has thought long and hard about Firefox's tab placement and has posted a video explaining the design decision (see below). The Mac OS and Linux versions will also see UI overhauls, though Mozilla's post doesn't go into details. Besides the changes listed above, other improvements can be expected before a final product is launched. App tabs are one of the features to be implemented and will allow users to make sticky tabs with no navigation interface for application-like websites (such as your email). This is sort of like permatabs. Firefox 4 Beta 1 is scheduled for release in the coming weeks. Mozilla plans to have the final version of the browser out the door by the end of the year. Via: Neowin Video: Why Tabs are on top in Firefox 4