Thunderbird works under Windows 8, just as it works under Windows 7.
Is Thunderbird available in the Windows Store?
There is currently no Thunderbird app for Windows 8, so you won't be able to use Thunderbird through the Modern (sometimes called Metro) user interface. It is a Desktop application.
Is Thunderbird compatible with Windows RT?
Since there's no Thunderbird app, Thunderbird won't run under Windows RT, since it only runs Windows Store apps.
So Thunderbird :
won't run on a Surface RT tablet, which has a Windows RT system, or any other Windows RT device.
will run on a Surface Pro tablet, which has the full Windows 8 system, and all other versions of Windows 8.
How can I install Thunderbird on Windows 8?
Simply click on the Desktop tile in the Start window. You will switch to the (mostly) familiar desktop environment, and there you can install and run Thunderbird.
Why does Thunderbird look like an old Windows 7 application on Windows 8?
Thunderbird does not have a new Theme for Windows 8. There is a Theme on the addon web site that is designed to give Thunderbird the look and feel of a windows 8 application. You can download the addon from here