According to the Firefox 2012 roadmap, Mozilla plans to introduce some major changes and new features this year, including a new default theme called Australis:

Firefox - default GNOME 3 theme

Firefox - Ubuntu (Ambiance theme)

Firefox - OSX

Firefox - Windows 7

As you can see in the images above, Firefox will use Chrome-like curved tabs on all platforms, the bookmarks star will be separated from the locationBar and merged with the bookmarks menu and apparently, the menu will be moved next to the locationBar. Further more, a new built-in download panel should finally replace the separate Firefox download window. There's already a Unfortunately, there are still no plans to support tabs in the titlebar for Linux (because of GTK limitations), but most other UI changes should be available for Linux too.As you can see in the images above, Firefox will use Chrome-like curved tabs on all platforms, the bookmarks star will be separated from the locationBar and merged with the bookmarks menu and apparently, the menu will be moved next to the locationBar. Further more, a new built-in download panel should finally replace the separate Firefox download window. There's already a custom Firefox build that comes with an initial version of the new download panel:

If you want to try the new theme, there's an unofficial Australis theme HERE (but it needs some work, at least under Linux). (but it needs some work, at least under Linux).

Besides the changes mentioned above, there are many other new features that should land in Firefox this year:

a new tab page (an initial version is already available in the latest Firefox Aurora builds - screenshot above) powered by the Awesomebar, with a new "speed dial" and new home tab which lets you access the Firefox preferences, etc.

Chrome migration

Web Apps (-webapp mode) and WebApps Marketplace integration

reworked session restore and a new Firefox restore which lets you restore bookmarks, passwords, etc.

Firefox Share: share links / update status across social networks

Firefox Login: user-centric site logins - one login for all the websites you us

in-line Preferences Manager

seamless plug-in install and update experience

built-in PDF viewer

more!

For more info, visit the Firefox roadmap page.